Effect of Dietary Iodine on Thyroxine Secretion Rate of Lactating Cows

Abstract

Supplemental iodine, 100mg 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 20mg 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 733kg higher than yields of supplemented cows in 28 weeks. There was no indication that the iodine deficient diet caused hypothyroidism in high producing cows.