Serum Thyrotropin, Thyroxine, and Tri-iodothyronine in Dairy Cows Fed Varying Amounts of Iodine

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Abstract
The objective was to determine whether dairy cows fed supplemental I daily at 2.5 or 5.0 mg/kg body wt for 49 wk had altered thyroid status relative to controls fed no supplemental I. Average daily dose of I was 1.6 and 3.3 g. Iodine as ethylene diamine dihydriodide was fed to Holstein cows beginning at 8-10 wk of lactation, for the remainder of that lactation, through the dry period, and into the next lactation. On day 341 after onset of I feeding, each cow was injected i.v. with 15 .mu.g thyrotropin releasing hormone per 100 kg body wt. 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 I/kg body wt; there were no differences between means. Releasing hormone increased thyrotropin, thyroxine and triiodothyronine in serum of all cows, but increases were not affected by quantity of dietary I.
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