Aspects of the absorption of oral L-thyroxine in normal man

Abstract

Using a double isotope method, the optimal conditions for the absorption of an L-thyroxine (L-T 4) preparation were examined. In particular, the influence of food intake on L-T 4 absorption was examined. As distinct from the unfavorable results hitherto obtained with L-T 4 lactose mixtures, a tablet similar to a commercial 100 μg L-T 4 lactose preparation showed good absorption of 70.6% ± 12.9% SD. In a corn starch preparation, 100 μg L-T 4 showed equally good absorption rates of 73.4% ± 10.8% SD. In the fasting state, the L-T 4 absorption of both preparations was significantly better ( p < 0.001) than with simultaneous food intake: 79.3% ± 7.2% SD versus 63.9% ± 10.5% SD. The calculated absorption of a 3-mg L-T 4 dose in a lactose preparation used in testing thyroid suppressibility was significantly lower ( p < 0.001) than the absorption of a smaller L-T 4 dose, and the absorption in the fasting state was again significantly ( p < 0.01) higher compared to that with simultaneous food intake: 52.6% ± 4.3% SD versus 43.9% ± 5.4% SD. The possibility of retention in the liver in the case of a large 3-mg L-T 4 dose is indicated. The data suggest that with L-thyroxine medication simultaneous food intake should be avoided.