Estimation of the Triiodothyronine Secretion Rate in Euthyroid Man*

Daily T3 secretion rates were estimated in 6 euthyroid subjects by administering increasing doses of triiodothyronine (T3) and utilizing suppressibility of thyroidal iodine release (TIR) as an endpoint. Daily T4 and T3 production rates were calculated so that T4 to T3 interconversion could be estimated. The estimated mean daily T3 secretion rate for the 6 subjects was 16 μg, while the T3 derived from peripheral interconversion was determined to be 12 μ/day. Therefore, approximately 58% of circulating T3 is derived from thyroid gland secretion and 42% from peripheral T4 to T3 interconversion. Since the mean daily T4 production was 92 μg, the amount of T4 metabolized via interconversion to circulating T3 would approximate 15%. The effects of pharmacological doses of T3 on TIR were also studied. It was noted that oral administration of 50 μg of T3 every 8 hr did not achieve full suppression of TIR. The addition of pharmacological doses of prednisolone, 20 mg every 8 hr to the high doses of T3, markedly augmented suppression of TIR. The TIR observed during administration of large doses of T3 alone is probably mediated via continued secretion of TRH, and may indicate that there is a positive feedback influence of thyroid hormone on the hypothalamic release of TRH.