Total and free thyroxine and triiodothyronine in blood serum of mammals.

1. Blood samples were obtained from seven species of mammals: horses, cattle, sheep, goats, pigs, guinea pigs and rats for determination of total and free thyroxine and triiodothyronine. Total thyroxine in the order listed above in ng/ml was: 15, 60, 79, 185, 53, 45 and 79. Free thyroxine in pg/ml was: 5.9, 10.0, 19.2, 32.1, 21.7, 6.7 and 51.3. 2. Total triiodothyronine in pg/ml was: 677, 1290, 979, 3170, 760, 317 and 1747. Free triiodothyronine in pg/ml was: 3.22, 4.40, 2.60, 6.74, 2.74, 2.42 and 10.88. 3. Percent free thyroxine was high in rats and low in guinea pigs, while percent free triiodothyronine was high in guinea pigs and low in goats. 4. Free thyroxine and percent free thyroxine were higher in some groups of horses, particularly stallions, than in other groups.