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Oral Effectiveness of L-Thyroxine, L-Triiodothyronine, and Thyroprotein as Compared to Injections of L-Thyroxine and Thyroprotein^{1, 2,}

Department of Dairy Husbandry, University of Missouri, Columbia, Missouri

ABSTRACT

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/100 lb bw, with a range from 0.15 to 0.25 mg/100 lb 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/100 lb bw, with a mean of 1.83 mg/100 lb bw.

L-T₃ given orally was 21.4% as effective as injected L-T₄, requiring a mean of only 0.8 mg/100 lb bw, with a range of 0.7 to 1.0 mg/100 lb bw.

Thyroprotein given orally was as effective as L-T₄ given orally. Doses of thyroprotein were given to contain from 1.25 to 2.25 mg L-T₄/100 lb bw, with a mean of 1.78 mg/100 lb 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 0.25 mg L-T₄/100 lb bw, with a mean of 0.21 mg L-T₄/100 lb bw.

¹ Contribution from Missouri Agriculture Experiment Station. Journal Series no. 2951 approved by the Director.

² Supplied by Agri.-Tech., Inc. Kansas City, Missouri.

³ Aided in part by a grant from U. S. Atomic Energy Commission Contract No. AT (11-1) 301-110.