42. Thyroid Gland

Question 1

Describe the structure of the thyroid gland.

Answer 1

The thyroid gland is a
highly vascular structure
made up of two lobes,
joined together by the thyroid isthmus.

The lobes are found
on either side of the trachea,

anterolaterally, below the larynx.

The isthmus passes in front of the trachea
overlying the second to fourth tracheal rings
in the adult.

At a cellular level,
the gland is made up of
thousands of follicles.

Each of these is made up of a single layer of cells surrounding a cavity. These
epithelial cells make thyroid hormones and secrete them into the cavity,
where they are stored bound to thyroglobulin, which is a globular colloidal
substance.

Question 2

What are the thyroid hormones and

Answer 2

There are three different thyroid hormones:

> Thyroxine (T4)

> 3,5,3-Triiodothyronine (T3)

> 3,3,5-Triiodothyronine (reverse T3)

The majority of hormone synthesised is T4,
but the active hormone is actually T3,
which is five times more potent.

T4 is converted to T3 peripherally
to cause its biological effect.

Reverse T3 is inactive.

Question 3

how are the thyroid hormones made

Answer 3

T3/T4 are made as follows:

1. > The thyroid gland takes in iodine
   by active transport
   and concentrates it here.
2. > This iodine is oxidised to
   atomic iodine by peroxidase.
3. > The atomic iodine
   iodinates tyrosine residues
   found on the thyroglobulin
   molecule to form mono– or di-iodotyrosine.
4. > These iodinated tyrosine residues
   then couple up to form either T3 or T4.
5. > The T3 and T4 hormones are stored
   as an integral part of the thyroglobulin molecule.

This process is driven
by thyroid-stimulating hormone (TSH)
acting via cAMP.

TSH also stimulates the release
of the hormones by driving the
endothelial cells to take in
the colloidal thyroglobulin by pinocytosis.

Once in the cell,
proteolysis of the molecule
causes release of T3 and T4.

These diffuse into the ready blood supply
and are transported out of the gland
bound mainly to T4-binding globulin,
but also to albumin and transthyretin.

Question 4

What are the effects of the thyroid hormones?

Answer 4

T3, the major active hormone, 
exerts its effects by 
combining with a receptor
in the cell nucleus 
and modulating protein synthesis 
at the level of the DNA.

The actions of thyroid hormone can be divided into:

Metabolic

1
• Increased basal metabolic rate by increasing the rate of oxidative metabolism

2• Increased sensitivity to catecholamines

3 •
Increased breakdown of proteins,
causing muscle wasting if unchecked

4 • Increased turnover of calcium from bone.

Growth

1. Needed for normal growth of tissues.

Thyroid hormones exert a direct effect
and also have a
permissive effect on growth hormone.

Nervous system

1 Needed for development of the nervous system
2 and normal myelination.

Others
Needed for normal gonadal function and for lactation.

Question 5

How are thyroid hormone levels controlled?

Answer 5

The hypothalamus produces
thyroid-releasing hormone,
which stimulates the release of
TSH from the anterior pituitary.

This in turn causes the release
of T4/T3 from the thyroid gland.

The T4/T3 released exerts a negative
feedback effect on the
hypothalamus and pituitary

thereby reducing further
release of stimulating hormones.

Ultimately, T4/T3 are
broken down in the liver
and most tissues.

T4 has a half-life of around 1 week,
while T3’s is much shorter at around 1 day.

Question 6

Hyperthyroidism

System Symptoms Signs

Answer 6

CNS

Irritable, change in behaviour,
anxiety, eye changes, goitre

Tremor, restless, irritable, frank psychosis in
severe cases, goitre, hyper-reflexia

Eye signs: exophthalmos (Graves’), lid lag,
ophthalmoplegia

CVS

Palpitations, racing heart

Tachycardia, atrial fibrillation, hypertension,
high-output cardiac failure, warm and
dilated peripheries

RS
Breathlessness
None specific

GI 
Weight loss despite increased intake, 
vomiting, 
diarrhoea
Weight loss

GU
Loss of libido, gynaecomastia

Oligo/amenorrhoea, gynaecomastia

Musculoskeletal Weakness, tremor, fatigue Proximal muscle wasting, tremor, palmar
erythema, pretibial myxoedema

Question 7

Hypothyroid

System Symptoms Signs

Answer 7

CNS

Fatigue, slowness of thought

Flat affect, deafness, frank psychosis in
severe cases, slow relaxing reflexes, ataxia

CVS

Ankle swelling Bradycardia, ischaemic heart disease,
peripheral oedema, low-output cardiac failure,
pericardial effusion (rare), hypertension,
vasoconstricted and cold peripheries

RS None None

GI Weight gain Weight gain, constipation

GU Menorrhagia Infertility

Musculoskeletal Thinning of hair, loss of
eyebrows, dry skin

Proximal myopathy, muscular hypertrophy,
myotonia