Effect of dietary iodine on estimated thyroid secretion rates of cows during lactation

Twelve Holstein cows were divided during their first month of lactation into two groups; (1) a control group consuming 4.4 mg per day iodine, and (2) a supplemented group fed an additional 100 mg of iodine per cow daily. All were fed 2.3 kg hay per day, com silage ad libitum, and averaged 13.4 kg per day of 18% crude protein concentrate containing 1% common NaCl. Production of milk, fat, total solids, and consumption of feed during the first 26 weeks of lactation were not significantly different between groups. Plasma protein bound iodine (PBI) and thyroxine secretion rate (TSR) were determined four times. Mean plasma protein bound iodine was 3.81 yg per 100 ml for Group 1 and 6.96 for Group 2 (P<.05). Mean thyroxine secretion, estimated from iodine utilization rate, was 1.96 mg per day for Group 1 and 2.53 for Group 2 (P<.05). Last period plasma protein bound iodine determinations averaged 84 and 59% thyroxine for the two grov.ps. Thus, actual thyroxine secretion rates for the groups were about equal. Milk iodine of Group 1 averaged 28.9 yg per liter compared to 319.6 for Group 2.