TSH Secretory Regulation: New Evidence that Triiodothyronine (T3) and Thyroxine (T4) Can Inhibit TRH Secretion Both in Vivo and in Vitro
The potential influence of thyroid hormones in vivo upon TRH secretion was examined using rat hypothalami in vitro derived from hyperthyroid (50 μg L-T4/100 g.B.W. x 4 days) and hypothyroid (4 weeks post total thyroidectomy) rats. Ouabain-activated TRH secretion from hypothalami derived from hyperthyroid animals was lowered in 14 + 3.7%, significant at p<0.02, and TRH secretion stimulated by ouabain was augmented 36 + 10% from hypo-thyroid hypothalami. Similarly, when L-triiodothyronine (T3) (0.1 μg/100 g.B.W.) was injected intraventricularly, serum TSH was lowered significantly without a concomitant elevation in peripheral T3 concentrations. In contrast, administration of the identical quantity of T3 i.p. had no suppressive effect upon TSH release. It is concluded that thyroid hormones regulate TSH secretion, at least in part, by inhibition of hypothalamic TRH secretion directly.
KeywordsThyroid Hormone Euthyroid Control Concomitant Elevation Hyperthyroid Animal Thyrotroph Cell
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