Thyroid Hormones in Blood and Milk of Dairy Cows

Abstract

Eighteen lactating holstein cows were selected with six in each group in, early, mid and late stage of lactation. Blood samples were obtained via tail vein on seven successive days. Blood serum and extracted milk samples were used by radioimmunoassay to measure concentrations of thyroxine, triiodothyronine, and reverse triiodothyronine. Serum thyroxine increased as stage of lactation progressed and milk production declined. Serum concentrations of triiodothyronine, reverse - triiodothyronine were unchanged at all stages of lactations. Concentrations of thyroxine and triiodothyronine in milk were significantly lower in early and mid but not in late stages of lactation. Reverse triiodothyronine in milk did not change.