EFFECTS OF HYPOTHALAMIC LESIONS AND OF COLD ON THYROID ACTIVITY IN THE RAT

ABSTRACT The effect was studied of an electrolytic basal midline lesion in the anterior part of the hypothalamus on thyroid activity of rats exposed to environmental temperatures of 24° C and 4° C. The index of thyroid function used was the biological half-life of thyroidal 131I calculated from the amount of radioactivity released by the gland in seven days. In one experiment, thyroid function was studied at 24° C one month after the placement of a lesion and at 4° C a month later. In another experiment hypothalamic lesions were made in the middle of a release study, during which some of the animals were kept at 24° C and others at 4° C. The two experiments yielded comparable results; the biological half-life values were approximately 7½ days at 24° C and 4 days at 4° C in intact or blank-operated rats, and approximately 13 days at 24° C and 7 days at 4° C in rats with a hypothalamic lesion. Thyroid 127I contents of various groups were identical. It appears that the biological half-life was reduced to approximately half the normal value by exposure to cold, that a lesion caused doubling of the half-life and that summation of the two effects was produced, when a lesioned rat was exposed to cold or when a cold-exposed animal was lesioned. The basal midline region of the hypothalamus destroyed by the lesions, whilst appearing to be essential for the maintenance of »spontaneous« thyroid activity, does not seem to be essential for the thyroid response to cold. Observations on so-called goitre-block lesions are discussed in the light of these findings.