Regulation of pituitary thyrotropin secretion.

The pituitary thyrotropin secreting cells of tetrapods share a common responsiveness to TRH, but the specificity of this response may differ among species. TSH responses in amphibians and reptiles appear to be somewhat less specific than in mammals, but each group differs in responsiveness to particular neuropeptides, and it is not clear that TRH is the main regulator of TSH in nonmammalian species. It is not yet known whether these diverse peptides are acting directly on the thyrotrope or possibly through a paracrine mechanism. A CRH-like peptide may be a common neuroregulator of the anuran thyroid and interrenal in metamorphosis. Inhibitory regulation including effects of neuropeptides and thyroid feedback at the pituitary level is evident in reptiles, but is not known for other nonmammalian groups. In ectotherms, the potential actions of temperature on TSH secretion must also be considered. In this regard, it is important to recognize that the thermal relations of the thyrotropin response may be distinct from other pituitary hormones, as well as other components of the hypothalamo-pituitary-thyroid axis (Licht et al., 1989).