Dominant inhibition of thyroid hormone action selectively in the pituitary of thyroid hormone receptor-β null mice abolishes the regulation of thyrotropin by thyroid hormone

Thyroid hormones, T-4 and T-3, regulate their own production by feedback inhibition of TSH and TRH synthesis in the pituitary and hypothalamus when T-3 binds to thyroid hormone receptors (TRs) that interact with the promoters of the genes for the TSH subunit and TRH. All TR isoforms are believed to be involved in the regulation of this endocrine axis, as evidenced by the massive dysregulation of TSH production in mice lacking all TR isoforms. However, the relative contributions of TR isoforms in the pituitary vs. the hypothalamus remain to be completely elucidated. Thus, to determine the relative contribution of pituitary expression of TR-alpha in the regulation of the hypothalamic-pituitary-thyroid axis, we selectively impaired TR-alpha function in TR-beta null mice (TR-beta(-/-)) by pituitary restricted expression of a dominant negative TR-beta transgene harboring a Delta337T mutation. These animals exhibited 10-fold and 32-fold increase in T-4 and TSH concentrations, respectively. Moreover, the negative regulation of TSH by exogenous T-3 was completely absent and a paradoxical increase in TSH concentrations and TSH-beta mRNA was observed. In contrast, prepro-TRH expression levels in T-3-treated TR-beta(-/-) were similar to levels observed in the Delta337/TR-beta(-/-) mice, and ligand-independent activation of TSH in hypothyroid mice was equivalently impaired. Thus, isolated TR-beta deficiency in TRH paraventricular hypothalamic nucleus neurons and impaired function of all TRs in the pituitary recapitulate the baseline hormonal disturbances that characterize mice with complete absence of all TRs.