Thyroparathyroid Uptake of Selenomethionine in the Rat

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Abstract
75Se-selenomethionine has been used clinically for parathyroid and pancreas scintiscannlng. This study analyzes the time sequence of selenomethionine uptake by the parathyroid and thyroid glands and the effects of thyroxine pretreatment and a low calcium diet on this uptake. The parathyroid uptake curve shows a peak in activity at 20 min. when the blood level is the lowest, resulting in a parathyroid to blood specific activity ratio of 2. This ratio decreases to unity at 2 hr. owing to a decline in parathyroid activity and a re-emergence of the label in plasma as protein. The parathyroid uptake increases 2-fold on a calcium-deficient diet. The parathyroid does not respond to thyroxine pretreatment. The activity of the thyroid is relatively constant over 4 hr. and never reaches the level of that of the parathyroid. The affinity of the thyroid gland for selenomethionine is markedly suppressed by thyroxine. This suppression is not altered by increasing the dose of thyroxine from 10 to 20 [mu]g and suggests that there are 2 components to the uptake of selenomethionine by the thyroid, only one of which is responsive to thyroxine pretreatment.

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