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Effect of Feeding Thyroxine on Secretion of I¹³¹ in Milk¹

Agricultural Research Laboratory of the University of Tennessee, Oak Ridge²

ABSTRACT

The metabolism of single doses of NaI¹³¹ was studied in eight comparisons with lactating cows fed 80 mg 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-labelled L-thyroxine. Thyroxine feeding, which began 2 wk before dosing and continued through seven-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 milliliter of milk did not differ significantly from that of controls. Thus, most of the greater excretion of I¹³¹ via the milk when thyroxine was fed was related to higher milk yield.

¹ This manuscript is published with the permission of the Director of the University of Tennessee Agricultural Experiment Station, Knoxville, Tennessee.

² Operated by the Tennessee Agricultural Experiment Station for the United States Atomic Energy Commission under Contract No. AT-40-1-GEN-242.