3,5-DI-IODO-L-THYRONINE SUPPRESSES TSH IN RATS IN-VIVO AND IN RAT PITUITARY FRAGMENTS IN-VITRO

3,5-Di-iodo-L-thyronine (T-2) is a naturally occurring metabolite of t hyroxine (T-4). Contrary to earlier findings, T-2 has recently been sh own to have rapid effects in rat liver and in mononuclear blood cells. In the experiments described here, T-2 was tested to determine whethe r it has a TSH suppressive effect in rats in vivo and in rat pituitary fragments in vitro. In experiments over 2 weeks in rats in vivo, low doses of T-2 (20-200 mu g/100 g body weight per day) had no significan t influence on body and organ weights, but significantly decreased TSH and T-4 serum concentrations. At 200 mu g/100 g per day, T-2 suppress ed TSH to 43% and T-4 to 29% of control levels. At 1-15 mu g/100 g per day, 3,5,3'-tri-iodo-L-thyronine (T-3), used as a comparison to T-2, had significant effects on TSH and T-4 levels, and also on body weight . Fifteen mu g T-3/100 g per day decreased TSH to 44%, T-4 to 25%, and body weight to 59% of control levels. In experiments over 3 months in rats in viva, a low dose (25 mu g/100 g per day) of T-2 suppressed TS H to 60% and T-4 to 57% of control levels and had no significant influ ence on other parameters. Conversely, 0.1 mu g/100 g per day T-3 had s ignificant effects on body and organ weights as well as pellet intake, but a less pronounced TSH suppressive effect: TSH concentrations were unchanged and T-4 concentrations were down to 80% of control values. In rat pituitary fragments in vitro, a clear suppression of TSH secret ion after a TRH pulse was demonstrated. To summarise, T-2 is a specifi c agonist in the negative feedback mechanism on TSH secretion at the p ituitary level without other apparent thyromimetic effects.