Further studies on response of blood thyroid hormone level to its acute depletion by isovolemic exchange transfusion.

Groups of rats weighing about 400 g and fed low iodine diet for 16 days were injected with 125I- daily from day 8 to 14. Two days later their blood thyroid hormone was acutely depleted with an isovolemic exchange transfusion of hormone-free blood cell suspension. Relative changes of total plasma radioactivity and iodinated compounds were measured at frequent intervals, maintaining isovolemia of the recipient animal. In normal animals a post-transfusion decrease of total plasma radioactivity was related to the degree of exchange of total blood volume as was the post-transfusion increase to the initial level. No post-transfusion increase of total plasma radioactivity or labeled thyroxine was found in thyreidectomized animals. On the other hand, in normal animals a post-transfusion increase of plasma labelled thyroxine was observed. A linear increase of labeled iodide in plasma was found in both normal and thyroidectomized rats, the mechanism of which in thyroidectomized animals may be explained by a reflux of iodide from peripheral tissues to the blood. In normal animals a TSH-induced release of iodide from thyroid due to TSH action may participate in this phenomenon.