Localization of human thyroxine absorption.

The distribution of intestinal absorption of 131I-labeled thyroxine (T4*) was studied in 4 normal subjects after oral and i.v. T4*, given in separate experimental sessions. In addition to collection of time-activity curves for plasma T4* from the two sessions, distribution and transport of T4* through the gut was quantified by external imaging. Time-activity curves were obtained for the stomach, duodenum, and upper jejunoileum. A multicompartmental model for systemic T4, with three distribution compartments and a single exit route, was employed. Additional, gastrointestinal, compartments were introduced. The stomach data were fitted to a model with three compartments, two for transport and a small sink of gastric activity that does not interact with the absorptive sites. Transfer from the duodenum to the upper jejunoileum and from the upper to the lower jejunoileum was modeled from fits to the peak T4* activities in the images of the duodenum and upper jejunoileum. The rate of transfer from the lower jejunoileum into more distal intestinal sites was fixed, but the impact on the results of using various values for this parameter was analyzed. The model calculations of absorption (mean +/- SD for 3 of the subjects) are duodenum, 15 +/- 5%, upper jejunoileum, 29 +/- 14%, and lower jejunoileum, 24 +/- 11%. The fourth subject, whose global absorption was abnormally low for uncertain reasons, had 17% absorption from the duodenum, 9% from the upper jejunoileum and none from the lower jejunoileum. Model projections mimicking clinical gut abnormalities known to affect T4 absorption were compatible with the results of published studies.