Ebook Fundamentals of biomedical science - Clinical Biochemistry: Part 2

12
Thyroid disease
Garry McDowell
Learning objectives
After studying this chapter you should be able to:
■

Describe the structure and function of the thyroid gland

■

Explain the function of thyroid hormones

■

Outline the action of thyroid hormones and control of their secretion from the thyroid
gland

■

Describe the conditions which lead to abnormal thyroid hormone production

■

Discuss the investigation of suspected thyroid dysfunction

Introduction
The thyroid gland secretes thyroid hormones that are required for normal metabolism of body
cells. Disorders of thyroid function can result in either inadequate or excess production of thyroid hormones causing altered cellular metabolism and development of associated clinical
features.
This chapter will describe the nature and role of thyroid hormones, their regulation in the

blood and the consequences of changes in their secretion. The value of laboratory investigations in diagnosis and monitoring of treatment will be discussed.

12.1

Structure of the thyroid gland

The thyroid gland is found below the larynx and is a butterfly shaped gland composed of a
right and left lobe on either side of the trachea. Both lobes are joined by an isthmus in front of
the trachea. The normal thyroid gland weighs approximately 30 g and is highly vascularized,
receiving 80–120 mL of blood per minute, as shown in Figure 12.1.


12.1 STRUCTURE OF THE THYROID GL AND

321

Hyoid bone
Common
carotid artery

Thyroid cartilage
of larynx

Internal jugular
vein

Right lateral lobe
of thyroid gland

Isthmus of

thyroid gland

Left lateral lobe
of thyroid gland

Trachea

Clavicle

Sternum

FIGURE 12.1
Anatomical location of the thyroid
gland in the neck.

Microscopic examination of thyroid tissues shows small spherical sacs called thyroid follicles
that make up most of the thyroid gland. The wall of each follicle is composed mainly of follicular cells, most of which extend to the lumen of the follicle. Figure 12.2 shows the structure
of thyroid follicles.
Follicular cell

FIGURE 12.2

Follicle containing
thyroglobulin

Histological structure of the thyroid gland
showing the follicles in which thyroid
hormones are made. Courtesy of Dr A L
Bell, University of New England College of
Osteopathic Medicine, USA.

322

12 THYROID DISEASE

A basement membrane surrounds each follicle. Follicular cells produce two hormones: thyroxine (T4), which contains four iodine atoms and tri-iodothyronine (T3), which contains three
iodine atoms. Together T4 and T3 are known as thyroid hormones. The parafollicular cells or
C-cells lie in between the follicles and produce a hormone called calcitonin, which regulates
calcium homeostasis.

SELF-CHECK 12.1

What are the two cell types in the thyroid gland and what hormones do they secrete?

12.2

Thyroid hormones

The thyroid hormones T4 and T3 are produced by the incorporation of iodine into tyrosyl
residues in thyroglobulin in a series of steps which are described as:

•
•
•
•
•
•
•
•

iodide trapping
synthesis of thyroglobulin
oxidation of iodide
iodination of tyrosine
coupling
pinocytosis of colloid
secretion of thyroid hormones
transport of thyroid hormones in the blood.

We will now consider each step in a little more detail. Figure 12.3 shows the steps involved in
the synthesis of thyroid hormones.
Follicular cells in the thyroid gland trap iodide ions by active transport from the blood into the
cytosol. The synthesis of thyroglobulin also occurs in the follicular cells. Thyroglobulin is a large
glycoprotein that is produced in the rough endoplasmic reticulum, modified by the attachment of a carbohydrate molecule in the Golgi apparatus and packaged into secretory vesicles.
The vesicles then release thyroglobulin in a process known as exocytosis into the follicle.
Thyroglobulin contains a large number of tyrosine residues that will ultimately become iodinated. In the diet, iodine is present in the form of iodide and this must be oxidized to iodine
which can be used for iodination of tyrosine residues of thyroglobulin. As iodide becomes
oxidized to iodine it passes across the cell membrane into the lumen of the follicle. As iodine
molecules form they are incorporated into tyrosine residues of thyroglobulin. The binding of
one atom of iodine to the tyrosine residues results in the formation of monoiodothyronine
(T1), whilst the binding of two iodine atoms results in the formation of di-iodothyronine (T2).
During the coupling step, two molecules of T2 join to form thyroxine (T4), while a coupling of
T1 and T2 results in tri-iodothyronine (T3). Iodinated thyroglobulin incorporating T4 and T3 is
stored in the colloid. Oxidation of iodide, iodination of tyrosine residues, and coupling reactions are all catalysed by the enzyme thyroid peroxidase. Then, under the control of thyroid
stimulating hormone (TSH) which is produced by the anterior pituitary, droplets of colloid
re-enter the follicular cells by a process known as pinocytosis and merge with lysosomes. The
enzymes present in lysosomes catalyse the proteolytic digestion of thyroglobulin releasing T4
and T3, whose structures are shown in Figure 12.4.

12.2 THYROID HORMONES

Colloid
Tyrosine

I2 I2
T3 I2

I2

I2 I
2
I2 T4

I–

Iodide

I2

Iodine

I2

TBG

I2 T2

T4

T3
I2

Thyroxine-binding
globulin

I2

T3

T3

T1

I2

I2

Thyroglobulin

I2

Oxidation of
iodide

T3

Follicular cells

I–

T4
Secretory
vesicles

I–

T4

Golgi complex

I–

I–

Rough
endoplasmin
reticulum

Active transport
of iodide
I–

I–
I–

I–

Lysosome

T3

I–

Synthesis of
thyroglobulin

Breakdown of
thyroglobulin
T3
T4

T3

T3

TBG
T4

I–

Blood

FIGURE 12.3
Synthesis of thyroid hormones T4 and T3.

Since T4 and T3 are lipid-soluble, they diffuse across the plasma membrane and enter the
circulation. Due to their lipophilic nature, more than 99% of T4 and T3 are bound to the transport protein thyroxine binding globulin (TBG). Thyroxine is released from the thyroid gland
in greater amounts than T3, although T3 is the more biologically active hormone. Thyroxine

enters cells and is deiodinated (removal of one I atom) to form T3.

T4
TBG

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324

12 THYROID DISEASE

I

HO

I

O

CH2CH

COOH

NH2
I

I

Thyroxine (T4)

I

HO

I

O

CH2CH

COOH

NH2
I

FIGURE 12.4
Chemical structures of T4 and T3.

Tri-iodothyronine (T3)

The majority of thyroid hormones in plasma are bound to specific proteins in order to render
them water-soluble, reduce renal loss, and to provide a large pool of hormones, whilst protecting the cells from the physiological effect of the hormone. The plasma binding proteins are
TBG and to a lesser extent albumin and pre-albumin. The plasma concentrations and proportions of thyroid hormones which are bound are shown below:

TBG
Pre-albumin
Albumin

Concentration

T4 (%)

T3 (%)

20 mg/L
0.3 g/L
40 g/L

70–75
15–20
10–15

75–80
Trace
10–15

The unbound or free T4 and T3 are considered to be the biologically active fraction that can
enter cells, bind to specific receptors, and initiate the physiological response and cause the
negative feedback regulation of thyroid hormone secretion.
The approximate reference ranges for serum concentrations of total and free thyroid hormones are:

T4
T3

Total

Free

60–160 nmol/L

1.2–2.3 nmol/L

10–25 pmol/L
4.0–6.5 pmol/L

Thyroxine is the major hormone secreted by the thyroid gland, which is converted by specific
de-iodinase enzymes, particularly in the liver and kidney, to form T3, the biologically active
hormone. The peripheral deiodination of T4 provides approximately 80% of plasma T3, the
remainder being derived from thyroid gland secretion.

SELF-CHECK 12.2

What are the steps involved in the synthesis of thyroid hormones?


12.4 CONTROL OF THYROID HORMONE SECRETION

325

TABLE 12.1 Effects of thyroid hormones on metabolic indices.
Increased by a rise in [thyroid hormone]

Increased by a decline in [thyroid hormone]

Basal metabolic rate

Plasma cholesterol

Plasma calcium

Creatine kinase

Sex hormone binding globulin

Creatinine

Angiotensin converting enzyme

Thyroxine binding globulin

Liver enzymes (gamma-glutamyl transferase)

Function of thyroid
hormones

12.3

Table 12.1 shows the effect of thyroid hormones on metabolism. They increase intracellular
transcription and translation, bringing about changes in cell size, number, and differentiation.
They also promote cellular differentiation and growth.
SELF-CHECK 12.3

What are the effects of thyroid hormones on metabolism?

Control of thyroid hormone
secretion

12.4

Hypothalamus

–

–
TRH

+

Thyroid hormone production is under both positive and negative feedback control as shown
in Figure 12.5.
Thyrotrophin releasing hormone (TRH) from the hypothalamus acts on the anterior pituitary
causing release of TSH, which in turn acts on the thyroid gland and stimulates the synthesis
and release of thyroid hormones. Briefly, a low blood concentration of free T4 or T3 stimulates the hypothalamus to secrete TRH, which enters the hypothalamic portal veins and flows
to the anterior pituitary where it stimulates thyrotrophs to secrete TSH. The TSH then acts
on the follicular cells to stimulate T4 and T3 production and their subsequent release. A
rise in the concentration of unbound T4 and T3 in the blood inhibits further release of TRH
and TSH from the hypothalamus and anterior pituitary respectively, via a negative feedback
effect.

–

Anterior pituitary

TSH

+

Thyroid gland

T4

T3

SELF-CHECK 12.4

What is the name given to the control mechanism where thyroxine controls its own
release?

FIGURE 12.5
Regulation of thyroid
hormone secretion.

–


326

12 THYROID DISEASE

12.5

Disorders of thyroid function

From a clinical perspective disorders of thyroid function can be classified into two broad categories: hyperfunction states where thyroid hormones are produced in excess, referred to as
hyperthyroidism, and hypofunction states where there is a deficiency of thyroid hormones,
referred to as hypothyroidism.

12.6

Hyperthyroidism

Hyperthyroidism has a significant short- and long-term morbidity and mortality. The prevalence of hyperthyroidism in women is ten times more common than in men. The annual
incidence of hyperthyroidism is quoted as 0.8/1,000 women.

Causes of hyperthyroidism
The most common causes of hyperthyroidism are Graves’ disease and toxic multi-nodular
goitre. Less commonly, hyperthyroidism may occur in patients on thyroxine therapy or due to
excess thyroid hormones being produced by ectopic thyroid tissue. Very rarely, hyperthyroidism may be a consequence of TSH secreting tumours.
Graves’ disease is an autoimmune condition characterized by the presence of diffuse thyroid
enlargement, eye abnormalities and thyroid dysfunction. The disease predominantly affects
females, with a peak incidence in the third and fourth decades of life.
Hyperthyroidism can often arise in patients with a multi-nodular goitre and occurs in an older
population than affected by Graves’ disease. The age of onset is typically over 50 years, with
females being affected more than males.
Drugs such as amiodarone can have a significant effect on thyroid function. Amiodarone is
used in the treatment of cardiac arrhythmias, has a structure similar to that of thyroid hormones, and interferes with the peripheral conversion of T4 to T3. Consequently the concentrations of T4 may be increased while T3 is low. In practice, it is advisable to check thyroid function
by assay of TSH and free T4 before commencing amiodarone treatment. Interpretation of
thyroid function test results can be problematic during treatment and assessment of thyroid
status during this time is best undertaken by careful clinical assessment.

Clinical features of hyperthyroidism
The clinical condition is often referred to as thyrotoxicosis and affected individuals present
with characteristic features. The common symptoms and signs of hyperthyroidism are shown
in Table 12.2.
On clinical examination of patients with Graves’ disease, a large and diffuse goitre is usually
present which is soft to the touch. A bruit is frequently heard over the thyroid and its blood
vessels due to increased blood flow through the hyperactive gland. Patients with Graves’ disease have characteristic eye signs, with a staring expression due to lid retraction, the white of
the eye or the sclera being visible above and below the iris. In addition there is a tendency for

12.6 HYPERTHYROIDISM

TABLE 12.2 Symptoms and signs of hyperthyroidism.
Symptoms

Signs

Increased irritability

Tachycardia

Increased sweating

Goitre

Heat intolerance

Warm extremities

Palpitations

Tremor

Lethargy

Arrhythmias

Loss of weight

Eye signs

Breathlessness

Proximal myopathy

Increased bowel frequency

Muscle weakness

the movement of the lid to lag behind that of the globe as the patient looks downwards from
a position of maximum upward gaze, referred to as ‘lid-lag’.
In patients with a toxic multi-nodular goitre, the cardiovascular features tend to predominate
in this often older population. The goitre is classically nodular and may be large.

Investigation and diagnosis of hyperthyroidism
Measurement of TSH will in most cases of hyperthyroidism show suppression of TSH to a
concentration below the lower limit of the reference range and in many cases to less than the
limit of detection for the assay. The exception to this is a TSH secreting pituitary tumour in
which case the concentration of TSH may be normal or at the top of the laboratory reference
range. Thyroid stimulating hormone secreting pituitary tumours, however, are extremely rare.
The concentration of free T4 is increased, often in association with a significant increase in free
T3 concentration. In some cases free T3 alone may be increased, with a normal T4 and low or
undetectable levels of TSH, and this is referred to as T3-toxicosis.
The diagnosis of Graves’ disease is made by the finding of hyperthyroidism on biochemical
testing, the presence of goitre, and extra-thyroidal signs such as eye signs. In other cases the
presence of a thyroid stimulating antibody (TSH receptor antibody) and diffuse increased
iodine uptake on thyroid scanning confirms the diagnosis.
The biochemical diagnosis of hyperthyroidism due to a toxic multi-nodular goitre is fairly
straightforward with suppression of TSH concentration. Free T4 and T3 concentrations are
increased although they may not be grossly abnormal, with values at or just above the reference range. Thyroid scintillation scanning shows patchy uptake of isotope with multiple hot

and cold areas being seen throughout the gland.
A TSH secreting pituitary adenoma is a rare cause of hyperthyroidism. In these cases TSH is
usually within the reference range, or inappropriately normal, or only slightly raised above
it, often around 6 mU/L, with an increased free T4 and T3. In such cases imaging will often
identify a pituitary lesion.
SELF-CHECK 12.5

What are the common clinical features of hyperthyroidism?

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12 THYROID DISEASE

Management of hyperthyroidism
The treatment of hyperthyroidism including Graves’ disease falls into three broad categories.
These are anti-thyroid drugs, radioactive iodine, or subtotal thyroidectomy. Some of the symptoms such as tachycardia and tremor can be controlled with β-blocking drugs for the first few
weeks of therapy. Radioactive iodine (131I) can be used to treat hyperthyroidism and works by
initially interfering with organification of iodine and then induces radiation damage to the thyroid. The major side effect of radioiodine treatment is that approximately 80% of subjects will
develop hypothyroidism as a result. There is no evidence of an increase in the risk of malignancy
following radioiodine therapy. Subtotal thyroidectomy is highly effective although surgical complications can occur in some patients. In elderly patients with a multi-nodular goitre, radioiodine
is the treatment of choice, although anti-thyroid drugs can be used until radioiodine treatment
becomes effective. Surgery may be required in patients who present with symptoms of hyperthyroidism and an enlarged thyroid gland compressing structures in the neck.
SELF-CHECK 12.6

What are the three broad categories of treatment for a patient with hyperthyroidism?

CASE STUDY 12.1

A 30-year-old housewife presented with weight loss, irritability, and had been feeling
uncomfortable whilst on holiday in Spain. She was taking oral contraceptive pills and
was not pregnant. On examination, her palms were sweaty, she had a fine tremor, and
there was no enlargement of the thyroid gland. The following results were obtained for
thyroid function tests (reference ranges are given in brackets):
TSH

<0.1 mU/L

(0.2–3.5)

Free T4

20 pmol/L

(9–23)

Free T3

22 pmol/L

(4.0–6.5)

(a) Comment on these results.
(b) What is the likely diagnosis?

CASE STUDY 12.2
A 40-year-old medical secretary attended for review of her treatment for Graves’ disease. She was taking carbimazole which was started one month previously. The results
for her thyroid function tests are given below (reference ranges are given in brackets):
TSH

<0.1 mU/L

(0.2–3.5)

Free T4

<5 pmol/L

(9–23)

Free T3

2.5 pmol/L

(4.0–6.5)

Comment on these results.


12.7 HYPOTHYROIDISM

12.7

Hypothyroidism

Hypothyroidism is an insidious condition with significant morbidity and the subtle and nonspecific signs are often associated with other conditions. Hypothyroidism is more common in
elderly women and ten times more common in women than in men. The annual incidence of
hypothyroidism is 3.5/1,000 women.

Causes of hypothyroidism
The causes of hypothyroidism can be primary where they affect the thyroid gland, or secondary where the anterior pituitary or hypothalamus is affected.
The most common cause of primary hypothyroidism is the autoimmune condition called
Hashimoto’s thyroiditis where autoantibodies cause progressive destruction of the individual’s own thyroid gland.
Loss of functioning thyroid tissue occurs following thyroidectomy or radioiodine treatment
and may lead to hypothyroidism. These patients will have an increased TSH concentration
with a low free T4 concentration, provided they are not receiving any form of thyroid hormone replacement therapy. Drug treatment with compounds such as lithium and iodine can
also result in hypothyroidism.
Other causes of hypothyroidism include congenital hypothyroidism, which occurs in newborn
children with a defect in the development of the thyroid gland, resulting in either its absence
or an undeveloped gland. Untreated children develop a condition referred to as cretinism.
Children with cretinism present with growth failure, developmental delay, and are often deaf
and mute. Box 12.1 gives further information about congenital hypothyroidism.

BOX 12.1 Congenital hypothyroidism
Congenital hypothyroidism is caused by a deficiency of thyroid hormones at birth, usually due to an absent thyroid gland or by an ectopic gland, which means the thyroid
gland is not in the correct anatomical position in the neck.
Congenital hypothyroidism in the UK occurs in approximately 1:3500 births. Most babies
with congenital hypothyroidism are diagnosed very early before symptoms develop by
means of the neonatal screening program, where thyroid hormones are measured in
a sample of blood collected on a special card from a heel prick. If signs and symptoms
are present, they may include feeding difficulties, sleepiness, constipation, and jaundice
(yellow colouration to the skin caused by excess bilirubin).
Children with congenital hypothyroidism are treated with thyroxine and placed on
life-long therapy. The prognosis is generally good and experience from the UK national
screening program has shown that almost all children with congenital hypothyroidism
who are diagnosed and treated early will mature normally.
A small proportion who have been diagnosed late or who have severe hypothyroidism may develop difficulties later in life such as poor hearing, clumsiness, and learning
difficulties.

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330

12 THYROID DISEASE

Diseases or injuries affecting the hypothalamus or anterior pituitary can result in reduced production of TRH and TSH respectively, causing a decline in production of thyroid hormones
from the thyroid gland. This is referred to as secondary hypothyroidism.

Clinical features of hypothyroidism
The clinical condition is often referred to as myxoedema and affected patients present with
features associated with reduced cellular metabolism. The common symptoms and signs of
hypothyroidism are shown in Table 12.3.
In parts of the world where there is iodine deficiency some patients may present with a goitre
and the thyroid gland undergoes hyperplasia. However, goitres also arise due to other reasons,
as given in Box 12.2.

Investigation and diagnosis of hypothyroidism
The routine biochemical assessment involves the measurement of TSH and free T4 concentration. As the concentration of thyroid hormones declines, the concentration of TSH increases.
The concentration of T3 is preferentially maintained and so measurement of T3 is not recommended as this could be misleading. Thyroxine concentration correlates better with thyroid
activity than that of T3 for diagnosis of hypothyroidism. A guideline for the interpretation of
thyroid hormone results is shown in Table 12.4.
Individuals with hypothyroidism due to Hashimoto’s thyroiditis will have an increased TSH concentration with low free T4 and the majority will have detectable thyroid antibodies. Thyroid
peroxidase antibodies may also be detected. The patient may also present with a history of
other autoimmune diseases such as diabetes, Addison’s disease, and pernicious anaemia.
Patients with secondary hypothyroidism will have a low serum TSH concentration together with a
low free T4. The distinguishing feature here is that the TSH concentration is inappropriately low.
SELF-CHECK 12.7

What are the common clinical signs of hypothyroidism?

TABLE 12.3 Symptoms and signs of hypothyroidism.
Symptoms

Signs

Lethargy

Periorbital and facial oedema

Dry coarse skin

Pale dry skin

Slow speech and mental function

Goitre

Cold intolerance

Cool peripheries

Pallor

Bradycardia

Hoarse voice

Median nerve compression

Constipation

Delayed relaxation of reflexes

Weight gain


12.7 HYPOTHYROIDISM

BOX 12.2 Goitre
A goitre is an enlarged thyroid gland and can mean that all the thyroid gland is swollen
or enlarged, or one or more swellings or lumps develop in a part or parts of the thyroid.
There are different types of goitre, such as:
■

Diffuse smooth goitre
This means that the entire thyroid gland is larger than normal. The thyroid feels
smooth but large. There are a number of causes. For example:
— Graves’ disease, an autoimmune disease which causes the thyroid to swell and
produce too much thyroxine
— thyroiditis (inflammation of the thyroid), which can be due to various causes, for
example viral infections
— iodine deficiency, the thyroid gland requires iodine to make T4 and T3
— some medicines can cause the thyroid to swell, for example lithium

■

Nodular goitres
— A thyroid nodule is a small lump which develops in the thyroid. There are two types:

— a multinodular goitre; this means the thyroid gland has developed many lumps
or ‘nodules’ and feels generally lumpy
— single nodular goitre, for example a cyst, an adenoma, or a cancerous tumour

Symptoms of goitre
In many cases there are no symptoms apart from the appearance of a swelling in the
neck. The size of a goitre can range from very small and barely noticeable, to very large.
Most goitres are painless. However, an inflamed thyroid (thyroiditis) can be painful.
There may be symptoms of hypo- or hyperthyroidism.
A large goitre may press on the trachea or even the oesophagus. This may cause difficulty
with breathing or swallowing.

Treatment of goitre
Treatment depends on the cause, the size of the goitre, and whether it is causing symptoms. For example, a small goitre that is not due to a cancerous nodule, when the thyroid
is functioning normally, may not require treatment. An operation to remove some or the
entire thyroid may be an option in some cases.

Management of hypothyroidism
Management of hypothyroidism involves the replacement of thyroid hormones, usually T4,
although T3 may sometimes be used. Treatment should be commenced carefully with elderly
patients, especially those with pre-existing ischaemic heart disease, being started on a low
dose and titrating the dose slowly. Thyroxine replacement therapy is monitored by regular
measurement of TSH and free T4. Adequate replacement is achieved when the TSH is within
the lower part of the reference range with a normal free T4. It should be noted, however, that
the concentrations of free T4 can vary post-dose, although this is not clinically significant.

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332

12 THYROID DISEASE

TABLE 12.4 Guide to the interpretation of thyroid function tests.
TSH low
T4 low

TSH normal

Severe non-thyroidal illness Sick euthyroid syndrome
Hypopituitarism
NSAIDs

TSH high
Hypothyroidism

Some anticonvulsants
TBG deficiency
Hypopituitarism
T4 normal

Thyrotoxicosis
Sub-clinical thyrotoxicosis
Treated thyrotoxicosis

Euthyroid
Adequate T4
replacement

Over-treated hypothyroid

T4 high

Thyrotoxicosis,
T4 replacement

Subclinical
hypothyroidism
Inadequate T4
replacement
Recovery from
non-thyroidal illness

Sick euthyroid syndrome
Erratic compliance with
T4 replacement

Erratic compliance
with T4 therapy.

Increased TBG

SELF-CHECK 12.8

How do you treat a patient with hypothyroidism?

CASE STUDY 12.3
A 63-year-old man, who was previously fit and well, presented with a five-day history
of shortness of breath associated with wheeze and dry cough. He denied symptoms of
hyperthyroidism and his family, social, and past medical history were unremarkable. The

electrocardiogram was consistent with atrial fibrillation and a fast ventricular response.
The results are as follows (reference ranges are given in brackets):
TSH

6.4 mU/L

(0.4–4)

Free T3

12.5 pmol/L

(4–6.5)

Free T4

51 pmol/L

(10–30)

Testosterone

43.1 nmol/L

(10–31)

FSH

18.1 IU/L

(1–7)

LH

12.4 IU/L

(1–8)

GH, prolactin and IGF-1 normal.
(a) Comment on these results.
(b) What further investigations would you suggest?
(c) Can you provide an explanation for these results?


12.8 L ABOR ATORY TESTS TO DETERMINE THE CAUSE OF THYROID DYSFUNCTION

Laboratory tests to determine
the cause of thyroid dysfunction

12.8

Thyroid peroxidase antibodies are present in about 95% of patients with autoimmune hypothyroidism secondary to Hashimoto’s thyroiditis. They may also be found in a small number
of healthy individuals but their appearance usually precedes the development of thyroid
disorders.
Thyroglobulin antibodies are found in many patients with autoimmune thyroid disease; however, measurement of thyroglobulin antibodies has no additional value to measuring thyroid
peroxidase antibodies alone.
Thyroid stimulating hormone receptor antibodies are measured in most routine laboratories
using methods that quantify the inhibition of TSH binding to porcine or human TSH receptors.
In most patients the measurement of TSH receptor antibodies is not essential for diagnostic
purposes.

The response of plasma TSH to a standardized challenge of infused TRH has been used
for many years to investigate patients with borderline hyperthyroidism. A marked TSH
response to >2 times the baseline value excludes hyperthyroidism. With the development
of new sensitive TSH assays it has been shown that a normal basal serum TSH predicts a
normal TSH response to TRH stimulation, whilst a suppressed basal TSH predicts a failure
to respond during TRH stimulation. The TRH test is now not routinely performed in clinical
practice. The measurement of free T4 and T3 is outlined in Box 12.3.

BOX 12.3 Measurement of free

T4 and T3

The measurement of TSH in a basal blood sample by a sensitive immunometric assay
provides the single most sensitive, specific, and reliable test of thyroid status. As we
have already discussed, the free hormones (free T4 and free T3) are widely held to
be the biologically active fractions. Direct methods involve measurement of the free
hormone in the presence of protein bound hormone. The analogue methods use
tracer derivatives of T4 or T3 capable of binding to the antibody but not reacting with
the binding proteins. The two-step assays involve the binding of the free hormone
in the sample with solid phase antibody, removal of the sample and back titration
of unoccupied binding sites on the antibody with labelled hormone. Interference in
free T4 and T3 assays by, for example, abnormal binding proteins and in vivo antibodies that bind T4 and T3, can cause problems in the interpretation of thyroid hormone
results.
The reference method for free T4 and free T3 measurement is equilibrium dialysis
using undiluted serum, but this cannot be performed in large numbers on a routine
basis.

333

334

12 THYROID DISEASE

Interpretation of thyroid
function tests
12.9

Interpretation of thyroid function tests can be difficult; however, there are a few basic principles which can help. Table 12.4 shows the most common causes of changes in the hormone
pairs TSH and free T4.
Pregnancy can have a significant effect on the result of thyroid hormone testing. In a normal pregnancy the concentration of TBG increases due to the action of oestrogen. Free
thyroid hormone concentrations also increase due to the weak thyroid stimulating effect
of high concentrations of human chorionic gonadotrophin (hCG) in early pregnancy. The
concentration of TSH is increased compared to the non-pregnant state, but remains within
the non-pregnant reference range. Hyperemesis gravidarum or a state of severe vomiting
during the first trimester is frequently associated with very high concentrations of free T4
and free T3 making it difficult to differentiate from true thyrotoxicosis. It is thought that very
high concentrations of hCG are also responsible for this condition.
Severe non-thyroidal illness can also affect the concentrations of thyroid hormones.
Interpretation of results should take into account the patient’s general clinical state and bear
in mind that during the illness and recovery the thyroid axis will not be in a steady state. A
general scheme for the interpretation of thyroid function tests is shown in Figure 12.6.

Tests of thyroid function
Normal thyroid hormones
Increased TSH

Symptomatic or
asymptomatic
+ve autoantibody

Thyroxine
therapy

Decreased TSH

Asymptomatic
–ve autoantibody

Repeat thyroid
function tests
3 months later

• Elderly subjects
• Euthyroid multinodular
goitre
• Previously treated
Graves’ disease or
ophthalmic Graves’
disease
• Corticosteriod therapy
• Early hyperthyroidism

Increased
thyroid hormones

Decreased
thyroid hormones

Normal or

increased TSH

Normal or
decreased TSH

TRH test
magnetic resonance
imaging of pituitary

Consider
thyroid hormone
resistance or
TSH secreting tumour

FIGURE 12.6
A flowchart for the interpretation of thyroid function tests.

Consider
sick euthyroid
syndrome
or hypopituitarism


12.9 INTERPRETATION OF THYROID
FURTHER
FUNCTION
READING
TESTS

SUMMARY

■

The thyroid gland produces hormones called thyroxine (T4) and tri-iodothyronine (T3)
which are required for normal cellular metabolism.

■

Release of T4 and T3 is controlled by thyroid stimulating hormone (TSH) produced by the
anterior pituitary, which in turn is controlled by release of thyrotrophin releasing hormone
(TRH) from the hypothalamus.

■

Disorders of thyroid function can result in either excess or reduced secretion of thyroid
hormones.

■

Hyperthyroidism occurs due to increased release of thyroid hormones and produces the
clinical features of thyrotoxicosis.

■

Hyperthyroidism can be treated with anti-thyroid medication, radioiodine, or surgery to
remove all or part of the thyroid gland.

■

Hypothyroidism occurs due to deficiency of thyroid hormones and produces the clinical
features of myxoedema.

■

Hypothyroidism can be treated by thyroid hormone replacement, usually with T4 alone.

■

Thyroid dysfunction can be investigated by measuring the concentration of serum TSH,
free T4, and free T3.

FURTHER READING
●

Association for Clinical Biochemistry, British Thyroid Association and British Thyroid
Foundation ( July 2006) UK Guidelines for the Use of Thyroid Function Tests. The
Association for Clinical Biochemistry, British Thyroid Association, and British Thyroid
Foundation. Available from the Association for Clinical Biochemistry.

●

Carson M (2009) Assessment and management of patients with hypothyroidism.
Nursing Standard 23, 48–56.

●

Cooper DS (2003) Hyperthyroidism. Lancet 362, 459–68.

●

Cooper DS (2005) Anti-thyroid drugs. New England Journal of Medicine 352, 905–17.

●

Dayan CM (2001) Interpretation of thyroid function tests. Lancet 357, 619–24.

●

Kharlip J and Cooper DS (2009) Recent developments in hyperthyroidism. Lancet
373, 1930–2.

●

Roberts CG and Ladenson PW (2004) Hypothyroidism. Lancet 363, 793–803.

●

Shivaraj G, Prakash BD, Sonal V, Shruthi K, Vinayak H, and Avinash M (2009) Thyroid
function tests: a review. European Review for Medical and Pharmacological Sciences
13, 341–9.

335


336

THYROID
12
THYROID
DISEASE
DISEASE

QUESTIONS
12.1

Which one of the following may cause hyperthyroidism?
(a)

Graves’ disease

(b) Hashimoto’s thyroiditis
(c)

Thyroidectomy

(d) Carbimazole
(e)
12.2

Cushing’s disease

The most common cause of primary hypothyroidism is:
(a)

Graves’ disease

(b) Hashimoto’s thyroiditis
(c)

Pituitary apoplexy

(d) Thyroid hormone replacement
(e)
12.3

Cushing’s disease

Patients with hypothyroidism may have a TSH result that is above the reference range.
(a)

True

(b) False
12.4

Which of the following methods can be used to treat hyperthyroidism? (select all that
apply)
(a)

Carbimazole

(b) Thyroxine
(c)

Surgery

(d) Insulin
(e)
12.5

Radioiodine

Which of the following proteins is the main plasma binding protein for thyroid
hormones?
(a)

Transferrin

(b) Thyroxine binding pre-albumin
(c)

Thyroxine binding globulin

(d) Caeruloplasmin
(e)
12.6

Fibrinogen

Which of the following is a symptom of hypothyroidism?
(a)

Heat intolerance

(b) Weight loss
(c)

Cold intolerance

(d) Palpitations
(e)

Increased bowel frequency


12.9 INTERPRETATION OF THYROID FUNCTION
QUESTIONS
TESTS

12.7

What is the most likely cause of the results below, obtained on a 25-year-old medical
secretary who is on 100 μg of thyroxine per day (reference ranges are given in
brackets)?
TSH: 7.7 mU/L (0.2–3.5)
Free T4: 25 pmol/L (9–23)

Answers to self-check questions, case study questions, and end-of-chapter questions
are available in the Online Resource Centre accompanying this book.
Go to www.oxfordtextbooks.co.uk/orc/ahmed/

337


13
Diabetes mellitus
and hypoglycaemia
Allen Yates and Ian Laing
Learning objectives
After studying this chapter you should be able to:
■

Describe the mechanism of glucose induced insulin secretion from the pancreatic β cell

■

Describe the control of blood glucose concentration by insulin and by the counter-regulatory
hormones glucagon, cortisol, adrenaline, and growth hormone

■

Identify the target tissues of insulin action

■

List the actions of the incretin hormones

■

Define insulin resistance and the metabolic syndrome

■

Describe the classical clinical features of and list the diagnostic criteria for diabetes
mellitus

■

Classify the different types of diabetes

■

Identify and classify the medical emergencies of diabetic ketoacidosis, hyperosmolar hyperglycaemic syndrome, and hypoglycaemia

■

List the long-term complications of diabetes

■

Identify treatment strategies for diabetes

Introduction
Diabetes mellitus is caused by an absolute or functional deficiency of circulating insulin,
resulting in an inability to transfer glucose from the bloodstream into the tissues where it
is needed as fuel. Glucose builds up in the bloodstream (hyperglycaemia) but is absent in
the tissues. The hyperglycaemia overwhelms the ability of the kidney to reabsorb the sugar
as the blood is filtered to make urine. Excessive urine is made as the kidney loses the excess
sugar. The body counteracts this by sending a signal to the brain to dilute the blood, which is
translated into thirst, expressed by frequent fluid intake called polydipsia. As the body spills


INTRODUCTION

glucose into the urine, water is taken with it, increasing thirst and the frequency of urination
(polyuria). Polydipsia and polyuria, along with weight loss (despite normal or increased food
intake) and fatigue (essentially because ingested energy cannot get to the tissues where it is
needed) are the classic symptoms of diabetes. One of the first to describe the disorder was
the ancient Hindu surgeon and physician Susruta, around 600 BC, who described a condition
‘brought on by a gluttonous overindulgence in rice, flour and sugar’ in which the urine is ‘like
an elephant’s in quantity’.

Key Points
Insulin deficiency can result from autoimmune attack and destruction of the insulin
secreting tissue (type 1 diabetes), or from the gradual overstressing of the insulin secreting tissue, due to a diet too rich in carbohydrate and fat and a lack of exercise (type 2
diabetes). The World Health Organization (WHO) has defined it on the basis of laboratory measurements of glucose.

As expressed graphically in Figure 13.1 the WHO estimated that in 1995 the worldwide prevalence of diabetes was 30,000,000 people, in 2005 it was 217,000,000, and by the year 2030 it
will be 366,000,000; a ten-fold increase in the world’s diabetic population in just 30 years. This
increase will be most prevalent in the developing world, in countries such as India and China.
Worldwide someone dies from diabetes-related causes every ten seconds, during which time
two other people will develop the condition. It ranks among the top three killer diseases along
with coronary heart disease and cancer. The treatment of diabetes and its complications will have
a significant impact on healthcare resources throughout the world for many years to come.
Figures for the UK are no less depressing. According to the WHO, there were 1.76 million
diagnosed diabetics in the UK in 2000 and it is estimated there will be 2.67 million by 2030.
The charity Diabetes UK estimates that there may be a further one million people with the
condition who haven’t been diagnosed. The UK national audit office calculated that from 1998
to 2008 the incidence of type 2 diabetes rose by 54%.
The already extensive economic burden diabetes puts on healthcare is set to rise further.
In 2002 the first Wanless Report estimated the total annual cost of diabetes to the NHS to
be £1.3 billion, with the total cost to the UK economy much higher. In 2004, Diabetes UK

FIGURE 13.1
Global growth in diabetes
(millions). See text for details.

339


340

13 DIABETES MELLITUS AND HYPOGLYCAEMIA

estimated that diabetes accounted for around 5% of all NHS spending. Approximately one in
every ten people treated in UK hospitals attend for treatment of diabetes and its complications. Consequently, regular attendance at diabetes and lipid clinics, and monitoring of cardiac
and renal function generate a significant workload for the laboratory.
SELF-CHECK 13.1

What are the four classic symptoms of untreated diabetes?

13.1

The islets of Langerhans

The human pancreas contains small groups of easily recognizable, specialized, endocrine
secretory cells surrounded by a sheath of collagen called the islets of Langerhans (Figure
13.2a), named after Paul Langerhans who first described them in 1869. The main cell type
within the islet is the beta (a) cell, which secretes insulin and comprises over 80% of the islet
mass (Figure 13.2b).
The alpha (`) cell secretes glucagon, the delta (c) cell secretes somatostatin, and the PP
cell secretes pancreatic polypeptide. Other cell types are also present in pancreatic islets, for
example ghrelin-secreting cells, which are involved in appetite (eat) signalling. Somatostatin
exerts an inhibitory paracrine effect on other islet endocrine cells, in addition to having several
extra-islet actions. A definitive function for pancreatic polypeptide has yet to be uncovered.

Glucose-induced insulin
secretion
13.2

Key Points

The a cells of the pancreatic islet act as glucose sensors; they secrete insulin in response
to rising levels of glucose in the bloodstream and they reduce insulin output in response
to falling glucose levels. This is known as stimulus-secretion coupling.

The mechanism of glucose-induced insulin secretion from the pancreatic β cell can be broken down into three main phases, namely transport and metabolism of glucose, metabolically

FIGURE 13.2
Section of human pancreas containing islets
stained with: (a) Haemotoxylin/eosin.
(b) Immunostained for insulin. Tissue section
and photomicrograph are courtesy of Ms
C Glennie and Dr G Howarth, Department of
Histopathology, Manchester Royal Infirmary, UK.


13.2 GLUCOSE-INDUCED INSULIN SECRETION

341

ΙΙ

(Electrical)
KATP
Ca++
VSCC

SUR1
X
KATP
channel

closure

Membrane
depolarization

K+

O
[Ca++]i

[K+]i

ATP
ADP

Secretory
granules

metabolism
Pancreatic β cell

GLUT 2
Glucose

Insulin

FIGURE 13.3

ΙΙΙ

Ι

(Secretory)

(Transport/metabolic)

generated changes in cellular ion flux, and finally the entry of calcium and the initiation of
calcium-dependent insulin release. We can see these phases in Figure 13.3.
Glucose enters the cell via a membrane-bound glucose transporter, known as GLUT 2
to initiate step 1 of the secretory mechanism. Once inside the cytosol, glucose is phosphorylated to glucose 6-phosphate by a high Km hexokinase enzyme called glucokinase
(or hexokinase IV). Unlike hexokinase I, II, and III, glucokinase has a high Km for glucose
(Km = 5.5 mmol/L), in other words it has a much lower affinity for glucose and can thus ‘sense’
this hexose over its physiological range. We can see this in Figure 13.4.

100

Enzyme activity (%)

Hexokinase
Glucokinase

50

FIGURE 13.4

0
0

5.0

10.0

Glucose (mmol/L)

15.0

Glucokinase vs hexokinase
activity. See text for details.

The three main phases of
glucose-induced insulin
secretion. See text for details.


342

13 DIABETES MELLITUS AND HYPOGLYCAEMIA

The expression and activity of glucokinase constitutes the rate-limiting step of stimulus secretion coupling and it has been called the β cell ‘glucose-sensor’. Genetic mutations in the gene
(GK) encoding this enzyme have been implicated in some types of diabetes as mentioned in
Section 13.7.
Once phosphorylated, glucose enters the glycolytic pathway to produce pyruvate, which is
further metabolized within the mitochondria. This in turn leads to an increase in generation of
the secretory signals NADH and ATP.
These metabolically generated signals initiate step II of the process, which involves the closure of a membrane-bound potassium ion channel. Beta cells are excitable cells and under
fasting conditions this channel is open and the membrane potential is maintained at around
−70mV by the relatively high intracellular to extracellular potassium gradient. The generation
of ATP from glucose metabolism causes a rise in the cytosolic ATP/ADP ratio, which closes this
nucleotide or ATP-sensitive potassium channel, the KATP channel. We can see this outlined
in Figure 13.3. Closure of this channel depolarizes the membrane to around −40 to −30mV.

This in turn opens a membrane potential-sensitive calcium channel in the plasma membrane,
the voltage-sensitive calcium channel (VSCC). As we can see in Figure 13.3, opening of the
VSCC facilitates the entry of extracellular calcium ions, allowing step III of the secretory process, calcium-induced, insulin secretion (Figures 13.3 and 13.5).
There are several operational ion channels in β cell membranes which could initiate depolarization in order to open calcium channels and generate calcium influx. Whilst the importance of the KATP channel cannot be understated, KATP channel-independent pathways for
glucose-stimulated insulin release also exist. One alternative pathway is via the activation of
the volume-regulated anion channel (VRAC), which is also operative in β cells. In terms
of depolarization, the loss of anions, or negative charge (for example chloride ions via VRAC
opening), or the build up of cations, or positive charge (potassium ions via KATP—closure) are
slightly different routes to the same outcome.
The first response to the influx of calcium is the exocytotic release of insulin granules from
stores close to the cell membrane (step III). This chemically primed pool of granules is called
the readily releasable pool. Its size determines the magnitude of the first phase secretory
response. For secretion to continue beyond this, granules must be mobilized from other stores
within the β cell. Increasing the cytosolic Ca2+ concentration initiates first phase secretion.
However, sustained insulin secretion can only be maintained if the cell is stimulated by metabolizable secretagogues such as glucose.

Pancreatic β cell

FIGURE 13.5
Ionic movements across a cells. This shows the principal cationic
fluxes in pancreatic beta cells, determined by normal ionic gradients
across the plasma membrane. The negative resting membrane
potential (-70mV) is a result of the relative greater outward K+
current rather than the combined inward Na+ and Ca2+ current.
Depolarization (i.e. a shift to a more positive membrane potential)
is achieved by reducing the K+ current (via closure of KATP channels)
upon stimulation of beta cells with glucose.

OUT
+

–
+
–
+
–
+
–
+
–
+
–
+

–
+
–
+
–
+
–
+
–
+
–
+
–

+
–
+

–
+
–
+
–
+
–
+
–
+

–
–
Ca++

–

–
+

–
–

+

–

+

–

+

–

–
–

+
–

(−70mV➜ −30mV)

–
–

–

–

–
–

–
( ) K+

–
–

–

Na+

IN
–

+

–
–

+

–


13.3 GLUCAGON SECRETION

SELF-CHECK 13.2

What is the correct sequence of events in glucose-induced insulin secretion?

Other insulin secretagogues
Substrates other than glucose can also initiate insulin secretion. These include leucine, ketoisocaproate, and methyl succinate. Other agents, called potentiators of insulin secretion,
have the ability to ‘amplify’ the effect of glucose on the β cell. Potentiators include some fatty
acids, the amino acid arginine, and the incretin hormones (mentioned in Section 13.5). The
sulphonylurea tolbutamide, a pharmaceutical used to treat type 2 diabetes, also has a direct
stimulatory effect on β cells. It acts by binding to a sulphonylurea receptor 1 (SUR1) found
on the plasma membrane of the β cell. This receptor is intimately linked with the KATP channel
(see Figure 13.3) such that when tolbutamide binds to its receptor, KATP channels close and the

cell depolarizes. Prandial glucose regulators also stimulate insulin release via the KATP channel as described in Section 13.11.

Insulin processing
Insulin biosynthesis starts from the translation of a single chain 86 amino acid precursor,
preproinsulin, from insulin mRNA, as we can see in Figure 13.6. As the molecule is inserted
into the β cell endoplasmic reticulum the amino terminal signal peptide is cleaved to form
proinsulin. In the endoplasmic reticulum the proinsulin is enzymically cleaved by several
endopeptidase enzymes to give insulin and what was the connecting peptide, c-peptide
(referred to as the C chain in Figure 13.6). Insulin consists of an aminoterminal B chain of 30
amino acids and a carboxyterminal A chain of 21 amino acids, which are connected by disulphide bridges occurring at cysteine residues in the protein. Insulin and c-peptide are packaged
in the Golgi apparatus into secretory granules. Zinc is also present in and released from the
secretory granule. In a normal individual insulin and c-peptide are co-secreted in a molar
ratio into the circulation. Both molecules are cleared from the bloodstream at different rates,
resulting in differing insulin to c-peptide ratios in the blood. In patients with type 2 diabetes, incomplete processing of the proinsulin molecule in the secretory granule results in the
release of various components of proinsulin (intact and split proinsulins) into the bloodstream.
Detectable concentrations of proinsulins have been observed in these patients. The peptide
hormone, amylin, is also co-secreted from the β cell. Amylin inhibits glucagon secretion,
delays gastric emptying, and acts as a satiety signal to the brain.

13.3

Glucagon secretion

In normal metabolism the concentration of circulating glucose is regulated by a balance
between the secretion of insulin and its opposing hormone, glucagon. Secreted from α cells
of the pancreatic islet, glucagon is one of the counter-regulatory hormones in glucose
homeostasis. Its secretion is influenced by a variety of different stimuli, including hormones,
nutrients, and neurotransmitters. Glucose is a potent physiological regulator of α cell function.
Insulin has been proposed as one of the main facilitators of glucose action on α cell activity,
and α cells are known to express large numbers of insulin receptors. Alpha cells also have a

KATP channel that is activated by zinc ions, which reduces glucagon secretion. Hyperglycaemia
rapidly suppresses glucagon release, whereas low blood glucose (hypoglycaemia) rapidly
facilitates glucagon secretion.

343


344

13 DIABETES MELLITUS AND HYPOGLYCAEMIA

N–ter

Signal
sequence

Chain B
S

S

S

S

S

S

Chain A

Preproinsulin

C–ter

Chain C
Signal
sequence
Chain B

N–ter
S

S

S

S

S

S

Chain A

C–ter

Proinsulin

C–ter

Insulin

Chain C

Chain C
Chain B

N–ter

S
N–ter

S

S

S

S
S

Chain A

C–ter

FIGURE 13.6
The steps involved in biosynthesis of insulin. See text for details. This figure is reproduced
with kind permission from the Beta Cell Biology Consortium (www.betacell.org), funded by
NIDDK U01DK072473.

Amino acids, such as arginine, non-esterified fatty acids (NEFAs), and ketones, suppress
glucagon secretion, as do insulin, zinc, and somatostatin. Stress hormones, such as adrenaline
and activation of the autonomic nervous system stimulate glucagon release. These effects are
especially pronounced during periods of stress, for example during hypoglycaemia, hypoxia,
and hypothermia, where readily available access to metabolic fuel can protect against potentially life-threatening situations. In healthy subjects, glucagon secretion is stimulated by a high
protein meal but inhibited by those rich in carbohydrate, or by oral glucose, helping to maintain blood glucose within the normal physiological range.
In diabetes there is a relative glucagon hypersecretion at normal and increased glucose concentrations and impaired responses to hypoglycaemia, resulting in a deterioration in glucosesensing by the α cell.