Effect of graded doses of thyroxine on the plasma TSH levels in mice.

The well known effect of T4 administration is suppression of TSH level in plasma; with small doses of T4 has been shown to potentiate goitrogenesis induced by goitrogenic drugs. The effects of hort term treatment with sm3.11doses of T4 on plasma and pituitary TSH levels were studied in mice, Very sm3.11doses of T4 caused an increase in circulating TSH while larger dose produced the usual decrease. Pituitary TSH levels decreased with the smallest dose employed; taken along with the larger plasma TSH level, this would indicate a greater release of TSH by pituitary, probablv meciiated though the hypothalamus , Increasing the dose of T4 caused an increase in pituitary TSH content, with a lowering of plasma TSH, probably inhibiting release whi1e synthe is is going on. With larger dose, T4 probably inhibited both synthesis and release of TSH as shown by the reduction of TSH levels in pituitary and plasma.