Reduced Biological Effectiveness of Orally Administered L-Thyroxine In the Rat in the Presence of Blood Plasma,

Abstract

Summary

The efficiency of absorption of an orally administered dose of L-thyroxine (L-T₄) in saline was found to be 48% compared to subcutaneous administration using the thyroid secretion rate (TSR) as an index. The addition of rat blood plasma to the L-T₄ before oral administration reduced its biological effectiveness to 34%, a significant reduction of 30% in comparison to oral L-T₄. These data suggest that the reduced biological effectiveness of oral L-T₄ may be due, in part, to a plasma-binding protein (TBP) in the intestines to which the L-T₄ becomes bound and fails to be absorbed.