Factors Affecting the Thyroid Uptake of I131 in Dairy Cows

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Abstract
The uptake of I131 was determined in a group of 8 cows, including 2 sets of identical twins, at approximately 6-week intervals for 1.5 years under varying conditions. The average peak uptake was 18% of the dose, and the average daily net release rate was 3% of the thyroid content. No significant differences were seen due to season or lactation per se. Dry twins had the same uptake as milking twins unless fed differently. Commercial dairy feed containing 10 mg I daily markedly inhibited I131 uptake. A special dairy feed containing 1.5 mg I daily also affected 1I131 uptake. Thyroid uptake of I131 was prevented by feeding 100 mg of l-thyroxine daily but only reduced 50% by injecting 3-5 mg of l-thyroxine daily. Uptake of I131 was resumed 7 days after stopping thyroxine feeding.

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