Abstract
The distribution and metabolism of thyroid hormones in the brains and the hypophyses of normal rabbits was determined after intravenous administration of I131-labelled triiodothyronine or thyroxine. Triiodothyronine concentrated in the gray matter of the brain, principally in the region of the paraventricular nucleus and the median eminence, and, more heavily, in the hypophyseal lobes. The pattern of distribution of thyroxine was similar, but concentration was much less intense. Very little activity was detected in the adenohypophysis, although the existence of an extravascular thyroxine space there was determined. Triiodothyronine rapidly left the plasma, but uptake in the tissues was slower, reaching a peak after about two hours. This pattern of concentration in time may have been dependent on the formation of a previously postulated plasma-triiodothyronine complex as a precursor of the tissue triiodothyronine. It resulted in high concentration gradients between tissues and plasma. Levels as high as 75:...
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