Free and total thyroid hormones in serum of Holstein cows.

Basic parameters are reported for free thyroxine and triiodothyronine in blood serum of dairy cattle. Free thyroxine ranged from a low of 1.47 ng/dl in the summer to 1.86 ng/dl in the spring and from 1.37 ng/dl in early lactation to 1.85 ng/dl in midlactation. The average free triiodothyronine went from a low of 2.98 pg/ml in winter to a high of 6.23 pg/ml in the fall and from 3.45 pg/ml in early lactation to 5.37 pg/ml in midlactation. The free fraction of the serum compared with the total fraction averaged .0267% for thyroxine and .291% for triiodothyronine. Free and total thyroxine were high in fall, low in winter, and intermediate in spring and summer. Free and total triiodothyronine showed a similar pattern with a greater magnitude in the variations. Even though the thyroid hormone secretion rate was higher in winter (earlier studies), increased tissue utilization kept serum concentrations low. Free and total thyroxine and triiodothyronine were low in early lactation due to high demands from peak milk production. Stage of lactation had a much greater influence on changes in serum thyroid hormones than did season.