In addition to stimulating TSH release, TRH is also a potent prolactin releasing factor (Fig. 1.14). The time course of response of blood prolactin levels to TRH, and the dose-response characteristics and suppressibility (by thyroid hormone pretreatment) of TRH, all parallel changes in TSH secretion. Nevertheless, there is reason to believe that TRH is not an important regulator of prolactin secretion under normal circumstances; for example, the prolactin secretory response to breastfeeding (in women and experimental animals) is unaccompanied by changes in plasma TSH levels, thus suggesting that this neurogenic reflex does not involve TRH. The prolactin release-stimulating effects of TRH, however, may be responsible for the occasional occurrence of hyperprolactinaemia (with or without galactorrhoea) in patients with hypothyroidism. This finding can be attrib uted to increased sensitivity of prolactin-secreting cells to TRH in the hypothyroid state, and to an increase in TRH secretion.
