triiodothyronine--reverse and isopropyl-diiodothyronine

The induction of mixed function hepatic oxygenases by rifampicin is known to increase the metabolic clearance rate (MCR) of T4. By performing T3 and rT3 kinetics we have shown that rifampicin also increases the MCR of T3 and rT3. Using the fall of serum T4 during TSH suppression as an indirect marker of the production rate (PR) of T4, we have demonstrated that there was no major change in monodeiodination nor any shift to either 5'- or 5-monodeiodination. Rifampicin stimulates in mice the mixed function hepatic oxygenases. However, we were unable to increase hepatic deiodinase activity (deiodinase type I) in this species. It is therefore possible that the increased MCR of T4 in man is not mediated by an increased conversion rate either. As mixed function hepatic oxygenases are known to increase hepatic conjugation it is suggested that rifampicin increases the biliary excretion of iodothyronine conjugates.

Topics: Adult; Animals; Diiodothyronines; Humans; Iodide Peroxidase; Kinetics; Liver; Male; Metabolic Clearance Rate; Mice; Rifampin; Thyroid Hormones; Thyrotropin; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

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.

Topics: Adult; Diiodothyronines; Feedback; Humans; Kinetics; Male; Metabolic Clearance Rate; Thyronines; Thyrotropin; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

Bromocriptine, because of its dopaminergic activity, could possibly inhibit TSH secretion. This hypothesis was tested in 7 normal male volunteers. Thyroid function was suppressed with the very potent thyromimetic analogue, 3'isopropyl-3,5-diiodo-L-thyronine (DIIP). We wanted to observe, after stopping this treatment, whether the return of serum T3, T4, rT3 and TSH values was influenced by concomitant bromocriptine treatment (5 mg/day). For comparison, the DIIP study was also performed without bromocriptine treatment. In both cases, serum T4 and serum T3, which had decreased by, respectively, 34% +/- 5% and 17% +/- 5%, returned over the same time interval to their initial values. Serum TSH and TRH-mediated TSH secretion were also unaffected by the bromocriptine treatment. DIIP does not interfere in the serum determination of T3, T4 and rT3. During suppression with DIIP serum T3 fell less than serum T4 and returned more rapidly to its initial concentrations. It is concluded that dopaminergic inhibition by bromocriptine, at this dose, is insufficient to alter normal thyroid function. In addition, during suppression and shortly thereafter there is a tendency for the serum T3 levels to be maintained.

Topics: Adult; Bromocriptine; Diiodothyronines; Homeostasis; Hormones; Humans; Male; Thyroid Hormones; Thyronines; Thyrotropin; Thyrotropin-Releasing Hormone; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse