Concentration of thyroid hormones and prolactin in dairy cattle serum and milk at three stages of lactation.

Eighteen lactating Holstein cows were used with six each in early, mid, and late lactation. Blood samples were obtained on 7 successive d. Blood serum and milk were measured by radioimmunoassay for thyroxine, 3,5,3'-triiodothyronine, and 3,3',5'-triiodothyronine. Prolactin was also measured in serum by radioimmunoassay. Serum thyroxine increased as lactation progressed and milk production declined (50, 55, and 62 ng/ml). Serum concentrations of triiodothyronine and reverse triiodothyronine were unchanged throughout lactation. Prolactin in serum declined as lactation advanced linearly (14.4, 11.8, and 10.5 ng/ml). Concentrations of thyroxine and triiodothyronine in milk declined significantly between early and mid but not mid and late lactation. Reverse triiodothyronine in milk did not change over the lactation. Serum triiodothyronine contained 1200 to 1300 pg/ml, whereas that in milk was 200 to 300 pg/ml. Reverse triiodothyronine was over 300 pg/ml in serum and only 80 to 90 pg/ml in milk. Amounts of thyroxine and triiodothyronine available to offspring from milk were calculated to be minor sources (4 to 5%) of total requirements for maintenance of metabolic function.