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

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Abstract
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.