TRH in the hypothalamus and the rest of the brain, as well as TSH and prolactin in the pituitary gland and the plasma have been determined by radioimmunoassay in rats varying in age (10- to 22-day-old fetuses and 1- to 60-day-old rats). TRH is first detected on the 16th day of gestation and its maximum increase occurs during the first 3 weeks of life both in the hypothalamus and the rest of the brain. The evolution patterns of TSH and prolactin in the plasma and the pituitary gland are discussed in relation to TRH levels in the brain and in the hypothalamus.

The effects of rapid changes of circulating cortisol levels on ACTH secretion and on corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) concentrations into hypophysial portal blood were studied in six adult rams. Pharmacological adrenalectomy was obtained by 3 h metyrapone infusion (100 mg·kg−1·h−1). Blockade of cortisol synthesis induced a tenfold increase of plasma ACTH levels accompanied by a moderate increase of CRF secretion (150% vs preinjection levels) and a large increment of AVP secretion (535% vs preinjection levels). ACTH levels remained high during the 3 h following the end of metyrapone infusion. During the same period, CRF secretion was still elevated (231% vs preinjection levels), while AVP secretion was further stimulated (2,151% vs preinjection levels). Subsequent hydrocortisone infusion (66 μg·kg−1·h−1) for 2 h induced a rapid decrease of both ACTH and AVP secretion, while CRF levels in hypophysial portal blood still remained elevated. These data suggest that changes in ACTH secretion induced by acute modifications of the negative glucocorticoid feedback are, in addition to the well documented direct effect of cortisol on the corticotropes, mainly mediated by variations of hypothalamic AVP secretion.

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In 12 patients with primary hypothyroidism the somatotrophic and corticotrophic functions were evaluated before and after thyroxine treatment. The results confirm a significant decrease, reversible by treatment, of plasma HGH responses to insulin-induced hypoglycaemia and to arginine infusion. Moreover, the results indicate that the impairment of the hypothalamic-pituitary function may also involve the response of plasma ACTH after provocative tests (insulin-induced hypoglycaemia and metyrapone). It must be stressed that the impairment of the corticotrophic function can be revealed only when the responses to provocative tests are evaluated by a direct assay of plasma ACTH and not by plasma corticosteroid modifications. These different responses may account for the conflicting results obtained by other investigators and may be justified by the multiple interference of the thyroid deficiency with the hypothalamic-pituitary-adrenal axis at different levels.

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