Serum Thyrotropin and Thyroxine after Thyrotropin Releasing Hormone in Dairy Cows Fed Varying Amounts of Iodine

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Abstract
The object was to determine whether dairy cows fed relatively large amounts of I had altered thyroid status. Polarographic analysis for determination of I and Br in milk and urine is described. Five dairy herds fed an average of 16 mg I/head per day (range 11-25) and 5 fed 164 mg I/head per day (range 74-402) were identified. I fed was estimated from minimal guaranteed I content of concentrates, iodized salt, minerals, liquid supplements and silage topdressing. I from other dietary sources was negligible. Each dairyman selected 3 cows each in early (< 90 days), middle (90-120 days) and late (> 120 days) lactation. Blood was collected from each cow selected, then 150 .mu.g thyrotropin releasing hormone were given. I in milk (.mu.g/ml) and urine (.mu.g/mg creatinine) were 2.2 and 65 for cows in high I herds, more than that (.37 and 20) in low I herds. At resting (ng/ml) serum thyroxine was 33 and 32, and thyrotropin was 7.4 and 7.9. Serum thyroxine and thyrotropin concentrations were increased following releasing hormone; increase was not affected by quantity of dietary I. Thyroxine in serum at resting (ng/ml) was less for cows in early lactation (25.9) than for those in middle (36.1) or late (35.5).