Chapter 9 The thyroid gland
Introduction
The thyroid gland secretes three hormones: thyroxine (T4) and triiodothyronine (T3), both of which are iodinated derivatives of tyrosine (Fig. 9.1), and calcitonin, a polypeptide hormone. T4 and T3 are produced by the follicular cells but calcitonin is secreted by the C cells, which are of separate embryological origin. Calcitonin is functionally unrelated to the other thyroid hormones. It has a minor role in calcium homoeostasis and disorders of its secretion are rare (see Chapter 12). Thyroid disorders in which there is either over- or under-secretion of T4 and T3 are, however, common.

Figure 9.1 Chemical structure of the thyroid hormones, T4 and T3, and the inactive metabolite of T4, rT3.
Thyroxine synthesis and release are stimulated by the pituitary trophic hormone, thyroid-stimulating hormone (TSH). The secretion of TSH is controlled by negative feedback by the thyroid hormones (see p. 119), which modulate the response of the pituitary to the hypothalamic hormone, thyrotrophin-releasing hormone (TRH; Fig. 9.2). This feedback is mediated primarily by T3 produced by the action of iodothyronine deiodinase on T4 in the thyrotroph cells of the anterior pituitary. Glucocorticoids, dopamine and somatostatin inhibit TSH secretion. The physiological significance of this is not known, but it may be relevant to the disturbances of thyroid hormones that can occur in non-thyroidal illness (see p. 160). The feedback mechanisms result in the maintenance of steady plasma concentrations of thyroid hormones.
Tests of thyroid function
Thyroid-stimulating hormone
Typical results of thyroid function tests in various conditions are shown in Figure 9.8.