Abstract
The diurnal variation in thyroidal iodine release previously observed in euthyroid subjects appears to correlate with variations in serum immunoassayable thyrotropin (TSH). The hypothesis is advanced that this diurnal rhythm seems to be primarily regulated by a negative feedback action of circulating hydrocortisone. The administration of maintenance doses of hydrocortisone to patients with primary adrenal insufficiency and pharmacological doses to euthyroid subjects was accompanied by an acute suppression in both thyroidal iodine release and serum TSH values. An escape from glucocorticoid suppression was observed to occur in 2 or 3 days with the resumption of a near-normal thyroidal iodine release rate but was accompanied by a dampening or absence of the normal diurnal rhythm. Withdrawal of pharmacological doses of glucocorticoids in euthyroid subjects and maintenance doses in primary hypoadrenal patients was accompanied by transient stimulation of both serum TSH and thyroidal iodine release values. The study of a patient before and after cryohypophysectomy indicated that the rebound response in thyroid release after steroid withdrawal may be a useful testing procedure to indirectly assess the hypothalamicpituitary reserve capacity of TSH.