Oral Effectiveness of l-Thyroxine, l-Triiodothyronine, and Thyroprotein as Compared to Injections of l-Thyroxine and Thyroprotein1,2

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The thyroid hormone secretion rate (TSR) of ten nonlactating dairy cows was determined using l-thyroxine (L-T4) and thyroprotein by injection. The equivalent biological effectiveness of L-T4, l-triiodothyronine (L-T3), and thyroprotein by oral administration was then determined.When L-T4 was injected, the mean TSR was found to be 0.17 mg/100lb bw, with a range from 0.15 to 0.25 mg/100lb bw. When L-T4 was administered orally, it was found to be only 9.4% as effective as by subcutaneous injection. The doses that inhibited thyroidal I131 release ranged from 1.5 to 2.0 mg/100lb bw, with a mean of 1.83mg/100lb bw.L-T3 given orally was 21.4% as effective as injected L-T4, requiring a mean of only 0.8mg/100lb bw, with a range of 0.7 to 1.0 mg/100lb bw.Thyroprotein given orally was as effective as L-T4 given orally. Doses of thyroprotein were given to contain from 1.25 to 2.25mg L-T4/100lb bw, with a mean of 1.78 mg/100lb bw. The biological effectiveness was 9.9%, very close to the 9.4% observed for l-thyroxine.When injected subcutaneously, thyroprotein was found to be only 84.4% as effective as injected L-T4. Thyroprotein was given to equal 0.15 to 0.25mg L-T4/100lb bw, with a mean of 0.21mg L-T4/100lb bw.