Is conversion of thyroxine to triiodothyronine and reverse triiodothyronine autoregulated? Studies with short term suppression of thyroid function with 3'isopropyl 3,5-diiodothyronine.

We investigated the effect of a 6 days course of 20 micrograms of a thyromimetic thyronine analogue 3'isopropyl, 3,5-diiodothyronine (DIIP) on serum thyroid hormone and TSH levels in 10 normal male volunteers. TSH was inhibited progressively during the 6 days of DIIP treatment. By the third day after stopping the treatment this trend had already reversed. Of the serum thyroid hormones the T3 level had decreased by 32% and was the first to increase 5 days after stopping treatment. Serum T4 fell by 35% and returned later to normal. The next effect was an increase in the T3/T4 ratio which was most marked 5 days after treatment had stopped. Kinetic studies with 125I-T3 and 125I-reverse T3 were performed at this time. In comparison to pretreatment kinetics no change in MCR was obtained. It is therefore suggested that at least part of the more rapid increase of T3 than T4 after stopping the suppression therapy reflects an increased conversion.