Serum thyrotropin, thyroxine, and tri-iodothyronine in dairy cows fed varying amounts of iodine.

The objective was to determine whether dairy cows fed supplemental iodine daily at 2.5 or 5.0 mg per kg body weight for 49 wk had altered thyroid status relative to controls fed no supplemental iodine. Average daily dose of iodine was 1.6 and 3.3 g. Iodine as ethylene diamine dihydriodide was fed to Holstein cows beginning at 8 to 10 wk of lactation, for the remainder of that lactation, through the dry period, and into the next lactation. On day 341 after onset of iodine feeding, each cow was injected intravenously with 15 microgram thyrotropin releasing hormone per 100 kg body weight. Thyrotropin in jugular blood collected before injection of releasing hormone averaged 7.6, 6.1, and 8.2 ng/ml for cows given 0, 2.5, and 5.0 mg iodine/kg body weight; there were no differences between means. Releasing hormone increased thyrotropin, thyroxine, and tri-iodothyronine in serum of all cows, but increases were not affected by quantity of dietary iodine.