Ultra short-loop feedback control of thyrotropin secretion.

Evidence is accumulating that pituitary hormone secretion is not only regulated by feedback from hormones produced in the target organs (long feedback) on the pituitary and the hypothalamus (feedforward), but also by a feedback of the hypophyseal hormones at the hypothalamic (short feedback) and the pituitary (ultra-short feedback) level. Inhibition of thyrotropin (TSH) and MSH secretion by pituitary preparations by adding exogenous TSH or MSH to the medium was already observed in the 1960s, as was the phenomenon that adrenocorticotropic hormone (ACTH) injected in the hypothalamus lowered plasma corticosterone levels. These early observations have now been corroborated by the demonstration of the receptors for various pituitary hormones in the hypothalamus and the adenohypophysis. The thyrotropin receptor (TSHR) is found on folliculo-stellate cells in the pituitary, which are known to influence the neighboring endocrine cells. This pituitary TSR-receptor is also recognized by TSHR receptor autoantibodies, which can downregulate TSH secretion independently from thyroid hormone levels, and are therefore thought to be responsible for the frequently observed suppressed TSH levels in patients with Graves' disease who are otherwise euthyroid.