3,5-Di-iodo-l-thyronine suppresses TSH in rats in vivo and in rat pituitary fragments in vitro

Abstract

Abstract 3,5-Di-iodo-l-thyronine (T2) is a naturally occurring metabolite of thyroxine (T4). Contrary to earlier findings, T2 has recently been shown to have rapid effects in rat liver and in mononuclear blood cells. In the experiments described here, T2 was tested to determine whether 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 T2 (20–200 μg/100 g body weight per day) had no significant influence on body and organ weights, but significantly decreased TSH and T4 serum concentrations. At 200 μg/100 g per day, T2 suppressed TSH to 43% and T4 to 29% of control levels. At 1–15 μg/100 g per day, 3,5,3′-tri-iodo-l-thyronine (T3), used as a comparison to T2, had significant effects on TSH and T4 levels, and also on body weight. Fifteen μg T3/100 g per day decreased TSH to 44%, T4 to 25%, and body weight to 59% of control levels. In experiments over 3 months in rats in vivo, a low dose (25 μg/100 g per day) of T2 suppressed TSH to 60% and T4 to 57% of control levels and had no significant influence on other parameters. Conversely, 0·1 μg/100 g per day T3 had significant effects on body and organ weights as well as pellet intake, but a less pronounced TSH suppressive effect: TSH concentrations were unchanged and T4 concentrations were down to 80% of control values. In rat pituitary fragments in vitro, a clear suppression of TSH secretion after a TRH pulse was demonstrated. To summarise, T2 is a specific agonist in the negative feedback mechanism on TSH secretion at the pituitary level without other apparent thyromimetic effects. Journal of Endocrinology (1995) 145, 291–297