Comparison of Biological Activity of Injected and Orally Administered L-Thyroxine, L-Triiodothyronine and Thyroprotein in Fowls.

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Abstract
Four-year-old male and female fowls of the New Hampshire strain were subjected to diurnal and seasonal variation in temperature. The T4 secretion rate (TSR) was 1st determined using L-T4, followed by L-T3 and thyroprotein (containing 1% L-T4) injected subcutaneously. In the male birds, if the mean TSR of L-T4 is considered 100%, L-T3 was observed to be 2.2 times as active biologically whereas thyroprotein was only 1.1 times as active. In the females, L-T3 was 1.5 times as active as L-T4, and thyroprotein was 1.3 times as active (based on 1% L-T4 content) as L-T4. In a comparison of subcutaneous and oral administration of these hormones in the male and using the subcutaneous administration as 100%, in 2 trials L-T4 orally was 96.2 and 88.8% as active. L-T3 was was 67.5% as active and thyroprotein, 58.2%. In the females in 2 trials, L-T4 was 40.7 and 68.7% as effective orally; L-T3 was 55.9% as effective orally; and thyroprotein was 62.1% as effective. Since the oral effectiveness of thyroprotein in male and females is about 60%, this value may be used in estimating the amount of thyroprotein to add to a poultry ration to equal the TSR.