Thyroid hormone-mediated regulation of corticotropin-releasing hormone messenger ribonucleic acid in the rat.

We have used in situ hybridization histochemistry to investigate interactions between the thyroid and adrenal axes in the rat. Propylthiouracil (PTU)-induced hypothyroidism caused a significant reduction in CRH gene transcripts in the paraventricular nucleus of male rats, with a concomitant decrease in both POMC gene expression in the anterior pituitary gland and circulating corticosterone. Conversely, adrenalectomy alone or adrenalectomy with high dose corticosterone pellet replacement had no significant effect on the thyroid hormone axis. The effect of thyroid hormones on the adrenal axis was not secondary to changes in the level of circulating corticosteroids, as the same effect of PTU-induced hypothyroidism was seen in adrenalectomized rats given corticosterone replacement. Neither was the effect of PTU simply the result of a direct toxic effect on the hypothalamus, as concurrent treatment with T3 blocked the response. We conclude that circulating levels of thyroid hormones have a major effect on the central regulation of the hypothalamic-pituitary-adrenal axis.