The thyroid gland produces two major active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). The production of these hormones within the thyroid is regulated by thyrotrophin (thyroid-stimulating hormone; TSH), a glycoprotein secreted by the anterior pituitary (Fig. 15.1). In turn, the production and release of TSH are controlled by thyrotrophin-releasing hormone (TRH), a tripeptide synthesised and released from the hypothalamus. Higher cerebral centres are also known to influence hypothalamic function. Circulating levels of T4 and T3 exert a negative feedback effect on TSH secretion via the pituitary and also the hypothalamus, whereby high levels of T4 or T3 suppress TSH, and low levels of T4 or T3 stimulate TSH secretion.
