Rate of Absorption of L-Thyroxine I131 from the Gastrointestinal Tracts of Dairy Cattle

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Abstract
Radlothyroxine was administered orally to 7 dairy cattle and the rate of absorption was measured by following the radioactivity of plasma samples. These samples were taken at intervals up to 96 hr after administration. The half-time (t 1/2) which can be defined as the time required for a substance to double in concentration or to decline one-half, was calculated from the radioactivity of the plasma. The absorption of thyroxine I131 from the gastrointestinal tract followed two phases, a rapid absorption phase with a t 1/2 of 2.65 hr and a much slower phase with a 11/2 of 20.1 hr. After this period, a declining phase with t 1/2 of 160.1 hr was observed as metabolism and excretion predominated. The recycling of I131 from metabolized thyroxine I131 was blocked by daily administration of a potent goltrogen, tapazole.