Biology of Disease

THYROID HORMONE DISORDERS

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Figure 7.18 Schematic to show the clinical

features of thyrotoxicosis.

The effects of thyroid hormones are to increase heat production, oxygen
consumption, the metabolism of proteins, fats and carbohydrates and to
promote normal growth. They are also necessary for the normal functioning
of the CNS. Levels of plasma T 4 and T 3 are regulated by the release of thyroid
stimulating hormone (TSH) from the anterior pituitary that, in turn, is
controlled by the release of thyrotrophin releasing hormone (TRH) from the
hypothalamus. Increasing concentrations of free T 4 and T 3 inhibit further
release of TSH and TRH by a negative feedback effect (Figure 7.17 (C)).

The most serious disorder of thyroid function is hyperthyroidism caused by
an excessive production of thyroid hormones. The clinical syndrome resulting
from hyperthyroidism is thyrotoxicosis (Figure 7.18). Its clinical features are
weight loss, sweating, heat intolerance, anxiety, hyperkinesis, increased
appetite, osteoporosis, menorrhagia, tachycardia and pretibial edema.

The commonest cause of hyperthyroidism is Grave’s disease, which can occur
at any age but particularly in 20- to 40-year-old females. Patients with Grave’s
disease suffer from exophthalmos or protrusion of the eyeballs, in addition to
clinical features of hyperthyroidism. It is an autoimmune disease (Chapter 5),
characterized by the presence of thyroid stimulating antibodies in the blood
that bind to TSH receptors in thyroid cells and stimulate them in a similar
manner to TSH. Toxic nodules are the second main cause of hyperthyroidism
and tend to be found in elderly patients. They may occur singly or as multiples
in a nodular goiter and are autonomous (self-governing) secretors of thyroid
hormones. An excessive intake of thyroxine in individuals who are treated for
hypothyroidism can cause hyperthyroidism. Rare causes of hyperthyroidism
include ectopic thyroid tissue and tumors that secrete TSH although the latter
are very uncommon.

Anxiety

Hair loss

Goiter

Tachycardia, palpitations

of the heart and atrial

fibrillation

Diarrhoea

Warm, moist palms

Tremors

and exophthalmos

General weight loss

Menorrhagia

Osteoporosis

Eye lid retraction