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

Abstract

Summary

Male and female fowls of the New Hampshire strain, about 4 years old, were subjected to diurnal and seasonal variation in temperature. The thyroxine secretion rate (TSR) was first determined using L-thyroxine (L-T₄), followed by L-triiodo-thyronine (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 two trials, L-T₄ orally was 96.2 and 88.8% as active. L-T₃ was 67.5% as active and thyroprotein, 58.2%. In the females, in two 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%, it is suggested that this value be used in estimating the amount of thyroprotein to add to a poultry ration to equal the thyroxine secretion rate.