Kinetics of Subcutaneous Absorption of L-Thyroxine-I131 as Measured by Increase of I131 in Blood of Cattle

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Abstract
Three hundred microcuries of I131 labeled thyroxine were injected subcutaneously into Holstein and Jersey cattle. At frequent intervals thereafter, I131 activity was measured both in the blood and in the thyroid gland. Half an hour after injection I131 could be detected in blood. The overall 24-hour absorption was exponential in character and could be described by a faster and a slower component. The range in half-time (t 1/2) for appearance of radio-activity in blood was 1.5-2.2 hours with a mean of 1.82 and 1.92 hours respectively for Holstein and Jersey cattle. In most of the animals the period of rapid increase in blood-I131was complete within 6-10 hours after thyroxine-I131 administration. Thyroidal-I131 determination indicated that I131 from the metabolism of thyroxine began to reach the thyroid gland within a period of 4 hours. The investigation revealed a gradual and yet sufficiently rapid increase in blood-I131 to simulate the normal physiological rate of thyroxine secretion. It is concluded that estimation of daily thyroxine secretion rate as determined by a replacement therapy method would not be subject to error by the rate of absorption of thyroxine from the skin.

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