Effects of Feeding Potassium Iodide, 3,5-Diiodosalicylic Acid, or L-Thyroxine on Iodine Metabolism of Lactating Dairy Cows

Three Jersey cows were fed daily 1.3 g iodine as KI, 1.3 g iodine as 3,5diiodosalicylic acid (DIS), or no supplemental iodine in three preiods of a Latin-square design. In each period, NaI ~ was administered orally twice daily for 21 days. The supplements were added to the capsules containing Nal ~ from the 8th through the I4th day. A single dose of I~-labelled L-thyroxine was injected intravenously on the 12th day of NaI ~ dosing. Iodine-125 excretion was only slightly affected by DIS, but KI caused a 34% reduction in milk I ~= and a 38% increase in urine I ~. Both compounds reduced excretion of I ~ from thyroxine into milk. Urinary I ~ excretion was highest from cows fed KI and lowest from those fed DIS. Neither supplement significantly influenced fecal I ~ or I ~ excretion. In another experiment, four cows were dosed orally with NaI ~ twice daily for 31 days. From the 11th through the 26th day, 80 mg of L-thyroxine daily were added to dosing capsules for three cows, and the fourth served as a control. Feeding thyroxine lowered I ~ concentration in milk 15%, but this was accompanied by greater milk yields. Total I ~:~ excretion in milk, urine, or feces was not changed by the thyroxine feeding.