Effect of Feeding Thyroxine on Secretion of I131 in Milk

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Abstract
The metabolism of single doses of NaI131 was studied in 8 comparisons with lactating cows fed 80mg L-thyroxine daily paired either with their identical twins or with their own performance in adjacent periods when no thyroxine was fed. Two additional cows fed thyroxine and their identical twin controls each received 1 mc I131-iabelled L-thyroxine. Thyroxine feeding, which began 2 weeks before dosing and continued through 7-day collection periods, increased milk production and excretion of I131 from both the iodide and thyroxine doses in milk, urine, and feces. Orally administered thyroxine increased rate of deiodination and plasma clearance of the thyroxine dose. Although more I131. from the iodide dose was secreted in milk of thyroxine-fed cows, the concentration of activity per ml of milk did not differ significantly from that of controls. Thus, it is indicated that most of the greater excretion of I131 via the milk when thyroxine was fed was related to higher milk yield.