“…There is now strong evidence that opiates can alter anterior pi tuitary hormone secretion by actions at the hypothalamus and /o r at the pituitary [3,4,15,20]. The effects of opiates on pituitary hormone secretion appear to vary with the endocrine status of the animal [2,7,8].The regulation of thyrotropin (TSH) secretion is influenced by opiates acting at the level of the pituitary a n d /o r hypothala mus [13,14,19,26], Opiates and endogenous opioid peptides (EOPs) have been demonstrated to have both an inhibitory [ 13,14,16,17,21,23,26] and stimulatory [14,17,22,23,26) effect on basal a n d /o r stimulated thyrotropin-releasing hormone (TRH) and TSH secretion.The hypothesis for the existence of a 'TSH center' or 'thyrostat' within the hypothalamus which is sensitive to circulating thyroid hormone and thus can secondarily control TSH secreReceived: January 31, 1990 Accepted after revision: March 21, 1990 tion by the pituitary is not new [32]; however, the role of brain opioid systems in regulating the activity of the thyrostat is un known. We have previously reported that for the luteinizing hormone (LH)-secretory mechanism, chronic opiate receptor stimulation with morphine increases the sensitivity of the hypo thalamus to the circulating concentrations of steroid hormones [7,8] and enhances both the positive ]8] and negative [7 ,8] feed back response of the 'gonadostat' to these steroids.…”