The effect of ambient temperature upon the secretion of thyroxine in the young pig.

1. Fluctuations in the rate of secretion and metabolism of thyroxine in the pig were observed whilst the ambient temperature was reduced from 32 to 8 degrees C. The quantity of food supplied was either maintained constant or doubled when the animals were exposed to the lower temperature. 2. It was found that the rate of disappearance of [125I]thyroxine did not change when the animal was exposed to the cold, unless the animal's food intake was altered. 3. The secretion of thyroxine increased temporarily after exposure to the cold. This was shown by a rise in the concentration of thyroxine in the plasma and a simultaneously increased loss of 131I from the thyroid gland. The increase in secretion of thyroxine was not maintained throughout the exposure to cold when the food intake of the animal was kept constant.