Thyroid hormones regulate the amount of thyrotrophin-releasing hormone in the hypothalamic median eminence of the rat.

The negative-feedback action of thyroid hormones on TRH in the hypothalamic median eminence was examined. Thyroidectomy caused a progressive decrease in TRH content of the median eminence, but not of the whole hypothalamus. In contrast, it did not affect the LHRH content of the median eminence. Administration of thyroid hormone prevented the decrease in TRH content of the median eminence after thyroidectomy. Destruction of the hypothalamic paraventricular nucleus (PVN) led to a significant reduction in TRH content of the median eminence in normal and thyroidectomized rats. These data provide evidence that thyroid hormones regulate directly the amount of median eminence TRH which is derived from the hypothalamic PVN in the rat.