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Rate of Absorption of L-Thyroxine I¹³¹ from the Gastrointestinal Tracts of Dairy Cattle^{1, 2,}

Department of Dairy Husbandry, University of Missouri, Columbia

ABSTRACT

Radiothyroxine was administered orally to seven 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) which can be denned 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 of 2.65 hr and a much slower phase with a t of 20.1 hr. After this period, a declining phase with a t 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 goitrogen, tapazole.

¹ Contribution from Missouri Agricultural Experiment Station. Journal Series No. 2452. Approved by the Director.

² Aided-in-part by a grant from U. S. Atomic Energy Commission, Contract No. AT(11-1)-301.