Efficiency of Absorption of Thyroxine in Various Forms from the Gastrointestinal Tracts of Lactating Cows

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Abstract
The efficiencies of absorption of orally administered crystalline L-thyroxine, thyroprotein, and thyroglobulin from the gastrointestinal tract of lactating cows were estimated by comparing the relative increase of plasma protein-bound iodine (PBI) from oral administration of these materials, with the corresponding PBI increases due to intravenous administration of L-thyroxine. In seven lactating cows the mean efficiency of absorption of thyroxine from thyroprotein (1.06% L-thyroxine) was 12.3%. Duplicate trials of absorption efficiency were performed in six other cows, the mean absorption efficiency being 13.9%; animal effects were highly significant statistically. Five lactating cows were used to determine the absorption efficiency of crystalline L-thyroxine and thyroxine in porcine thyroglobulin concentrate (0.37% L-thyroxine). Again, highly significant differences were found among animals. The mean absorption efficiencies of the crystalline L-thyroxine and the thyroxine from thyroglobulin were 11.6 and 15.9%, respectively. These results suggest that one of the important causes of animal variation in their lactational (or other) responses to orally administered thyroidal materials may be the efficiency with which the materials are absorbed from the digestive tract.