Altered response of blood thyroxine level after its acute depletion by isovolemic exchange transfusion in rats with lesions in hypothalamus and thalamus

Abstract

Bilateral electrolytic lesions were made in various areas of hypothalamus or thalamus on the 6th day of a period of daily radioiodide injections (1 or 5 muCi125I-daily per animal) in male rats weighing about 350 g. Such injections were continued for another 4 days and after 2 days of intermission the blood thyroid hormone was acutely depleted by isovolemic exchange transfusion of thyroid hormone free blood cell suspension. Relative changes of plasma thyroxine level were measured with the aid of paper chromatography in small aliquots of plasma frequently taken from the animals under maintaining isovolemia by replacing the removed plasma. It was found that in animals with various bilateral electrolytic lesions in hypothalamus (suprachiasmatic and paraventricular areas) the response of blood thyroxine level after the transfusion is similar as in sham-operated controls bearing unilateral subcortical lesion or in normal animals observed previously. On the other hand, the response in animals with thalamic lesions was repeatedly found to resemble that observed previously in thyroidectomized animals. Since the response of blood thyroxine level presumably results from changes of pituitary thyrotropic activity, it is concluded that in rats with thalamic lesions the normal response of hypothalamo-pituitary-thyroid axis was prevented. The mechanism of this action, however, remains to be elucidated.