Decreased central serotonin function in hypothyroidism.

Male rats born of mothers kept on an iodide-free diet from day 15 of pregnancy up to day 4 post-delivery, were rendered hypothyroid by a daily s.c. injection of methimazole, 20-25 mg/kg from day 1 up to day 30 of age. Two days after the last injection, euthyroid and hypothyroid rats were given 220 mg/kg i.p. of L-5-hydroxytryptophan (5-HTP). The resulting behavioral syndrome was much less intense in hypothyroids, compared to euthyroids, thus suggesting a state of serotonergic hypoactivity.