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, T₄ and T₃, regulate their own production by feedback inhibition of TSH and TRH synthesis in the pituitary and hypothalamus when T₃ 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-α in the regulation of the hypothalamic-pituitary-thyroid axis, we selectively impaired TR-α function in TR-β null mice (TR-β^-/-) by pituitary restricted expression of a dominant negative TR-β transgene harboring a Δ337T mutation. These animals exhibited 10-fold and 32-fold increase in T₄ and TSH concentrations, respectively. Moreover, the negative regulation of TSH by exogenous T₃ was completely absent and a paradoxical increase in TSH concentrations and TSH-β mRNA was observed. In contrast, prepro-TRH expression levels in T₃-treated TR-β^-/- were similar to levels observed in the Δ337/TR-β^-/- mice, and ligand-independent activation of TSH in hypothyroid mice was equivalently impaired. Thus, isolated TR-β 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.