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Effect of Dietary Iodine on Thyroxine Secretion Rate of Lactating Cows

Department of Animal Science, University of Tennessee, Knoxville 37916

ABSTRACT

Supplemental iodine, 100 mg daily, was added to low iodine diets of seven Holstein cows which were compared to paired controls. Treatment was from 6 to 8 weeks prepartum through 28 weeks of lactation. Plasma thyroxine (T₄) and T₄ disappearance rates after injection of 20 mg T₄ were determined four times at 2-month intervals. Iodine supplemented cows averaged higher than controls in plasma total iodine, protein-bound iodine, and T₄. Average differences in T₄ turnover rates were not significant, but overall T₄ secretion or disappearance rates were higher for the supplemented cows in the last two periods. Plasma thyroxine secretion rates of both controls and iodine supplemented cows increased slightly with advancing lactation. Incipient iodine deficiency was indicated in the control cows at the end of the trial by low milk iodine and blood iodide concentrations. Milk yields of control cows averaged 733 kg higher than yields of supplemented cows in 28 weeks. There was no indication that the iodine deficient diet caused hypothyroidism in high producing cows.