Effects of serotonergic system on hypothalamic-pituitary-thyroid axis in rats

The effects of the serotonergic system on the hypothalamic-pituitary-thyroid axis in rats were studied. Immunoreactive thyrotropin releasing hormone (ir-TRH) in plasma significantly increased 24 hours after para-chlorophenylalanine (PCPA) administration. Basal thyrotropin (TSH) levels in plasma significantly decreased after tryptophan or 5-hydroxytryptophan (5-HTP) administration, but not after PCPA administration. The plasma ir-TRH and TSH responses to cold were inhibited after tryptophan or 5-HTP administration, but not after PCPA treatment. In these treated groups, the TSH response to TRH did not differ from that of control. Thyroxine, 3,3',5-triiodothyronine showed no change after administration of these drugs. These data indicate that the serotonergic system might inhibit TRH and TSH release in rats.