The significance of deep body temperature in regulating the concentration of thyroxine in the plasma of the pig.

1. The activity of the thyroid gland in the pig has been assessed while thermosensitive regions of the hypothalamus or the spinal cord were cooled.2. If the cervical region of the spinal cord was cooled for 3 hr, by means of a thermode in the epidural space, the concentration of thyroxine in the plasma increased to a maximum within the first hour but fell again to the control level or lower within 2 hr of the onset of cooling.3. When a thermode in the preoptic region of the hypothalamus was cooled to 18 or 25 degrees C for 3 hr, the concentration of thyroxine in the plasma increased within the first hour and then declined again. Cooling the thermode to 10 degrees C led to a sustained increase in thyroxine concentration.4. The increase in the concentration of thyroxine in the plasma, on cooling the hypothalamus, was of a similar magnitude whether the ambient temperature was 25 or 15 degrees C.5. Observations on the disappearance of radioactive thyroxine from the plasma indicated that the distribution or metabolism of thyroxine was altered by the cooling of either the hypothalamus or the spinal cord.6. It was concluded that when the preoptic region of the hypothalamus or the cervical region of the spinal cord was cooled the rise in the concentration of thyroxine in the plasma was due at least in part to a change in the metabolism or distribution of thyroxine within the body and not entirely to the stimulation of the thyroid gland as has previously been suggested.