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 thyroxine (T 4 ). Contrary to earlier findings, T 2 has recently been shown to have rapid effects in rat liver and in mononuclear blood cells. In the experiments described here, T 2 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 T 2 (20-200 μg/100 g body weight per day) had no significant influence on body and organ weights, but significantly decreased TSH and T 4 serum concentrations. At 200 μg/100 g per day, T 2 suppressed TSH to 43% and T 4 to 29% of control levels. At 1-15 μg/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 μg 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 vivo, a low dose (25 μg/100 g per day) of T 2 suppressed TSH to 60% and T 4 to 57% of control levels and had no significant influence on other parameters. Conversely, 0.1 μg/100 g per day T 4 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 T 4 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, T 2 is a specific agonist in the negative feedback mechanism on TSH secretion at the pituitary level without other apparent thyromimetic effects