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

Summary

A comparison of the biological activity of L-T₄, L-T₃, and thyroprotein (a synthetic thyroaetive iodinated casein guaranteed to contain 1% L-T₄), when administered orally and by subcutaneous injection, is reported. The comparative activity was measured by the thyroid secretion rate (TSR) technique using the same groups of Sprague-Dawley-Rolfsmeyer strain of male rats for each comparison. Using the subcutaneous administration of L-T₄ as a standard, it was shown that L-T₄ given orally was 37.9% as effective and L-T₃ was 95% as effective as L-T₄ on an equimolar basis when administered orally. Since L-T₃ is 2.6 times as active as L-T₄ by injection, its oral effectiveness would be 36.9%. Thyroprotein, on the basis of 1% L-T₄, was 36.2% effective orally.