STUDYOF THYROID STIMULATING HORMONE (TSH) SECRETION INDUCED BY CONTINUOUS THYROTROPIN RELEASING HORMONE (TRH) STIMULATION

TSH secretory dynamics under continuous TRH stimulation for two or three hours was studied in euthyroid and hypothyroid rats.In euthyroid, plasma TSH level induced by continuous TRH stimulation was the biphasic pattern, which had two peakes of response around 15 minutes (min) and between 90 to 120min, and the nadir at 60min after TRH stimulation. Changing the dose of TRH at 45min, bigger TSH response was observed after 60min in higher TRH dose than in ordinary TRH dose. Thyroxine (T4) pretreatment had no effect on the biphasic TSH response.In hypothyroid made by methylmercaptoimidasole treatment (MMI), plasma TSH level induced by TRH was the monophasic, which had one single peak around 30min and then kept the pre-experimental basal level. In MMI rats maintained with T4 (MMI+T4), the biphasic TSH response to TRH was seen again.Pituitary TSH content of euthyroid was almost same as of MMI+T4, and they both were higher than that of hypothyroid, significantly.Conclusion : In euthyroid, continuous TRH stimulation induced the biphasic TSH response. The nadir of TSH response was likely due to the transient desensitization of TRH receptor. In hypothyroid, TSH response to TRH was monophasic. Pituitary TSH content seemed to make the difference of TSH response between in euthyroid and hypothyroid.