Thyroxine feed-back on the regulation of thyrotropin [TSH] secretion.

1. The role of thyroid hormone as a feed-back at the hypothalamic level in the control of thyrotrophin (TSH) secretion and release has been investigated by estimating the plasma and pituitary TSH levels following intrahypophysial and intrahypothalamic thyroid autotransplants.2. Thyroidectomized rats bearing thyroid autotransplants in the pituitary had a significantly lower (P < 0.001) plasma TSH than that of controls at 26 degrees C but not at 4 degrees C.3. Thyroidectomized rats bearing thyroid autotransplants in the supraoptic area showed a significantly lower (P < 0.001) level of plasma TSH and higher pituitary TSH at 4 degrees C but not at 26 degrees C.4. Study with both unilaterally and bilaterally thyroidectomized rats bearing thyroid autotransplants either in the pituitary or in the hypothalamus revealed that thyroxine feed-back operates at pituitary level in normal situations (26 degrees C) and there exists a feed-back through higher centres, specifically the TSR secreting area of the hypothalamus, in situations demanding higher thyroid function, as in cold exposure.