Role of thyrotrophin-releasing hormone in the development of pituitary-thyroid axis in four anencephalic infants.

Anencephaly provides a unique model for studying endocrine functions in absence of hypothalamic influence. We previously reported that in anencephalic newborns both pituitary TSH-secreting cells and the thyroid were normal and were able to function if adequately stimulated. In order to verify if the normal development of the pituitary-thyroid axis in these infants depends on TRH of extrahypothalamic origin, we measured endogenous TRH levels in the clear fluid of a cyst of the cerebro-vasculosa in 4 anencephalic newborns. In these cysts were also injected 200 micrograms of synthetic TRH and evaluated TSH response in peripheral blood samples. Endogenous TRH was detectable in the cysts of the cerebro-vasculosa in 3 of the 4 infants. In all 4 cases serum TSH sharply increased after TRH administration. Our data suggest that the normal development of the pituitary-thyroid axis in anencephalic infants either requires no TRH or depends on extrahypothalamic TRH. In this latter case TRH produced by other areas of the central nervous system might be secreted into the cysts of the cerebro-vasculosa, actively transported to the hypophyseal vessels, and might thus reach the pituitary to stimulate TSH-cells growth and function.