Distribution of Subcutaneous Thyroxine, Triiodothyronine, and Albumin in Man: Comparison with Intravenous Administration Using a Kinetic Model*

Distributional kinetics of radioiodinated T4 (T4), T3 (T3), and albumin (RISA), after simultaneous administration by the sc and iv routes of 125I- and 131I-labeled compounds, were measured in normal subjects. Data were analyzed by fitting them, using the SAAM technique, to models with three compartments for the iv administered compounds and a fourth compartment for the site of the sc injection. The radiopharmaceuticals administered sc transfered by first order kinetics from the injection site to plasma with half-lives of 20.4, 5.6, and 63.0 h for T4, T3, and RISA, respectively. Percentages of 3.3, 1.2, and 2.1 of the sc dose appeared directly in the vascular compartment. In some, but not all, studies with sc T4 and RISA, a portion of the disappearance from the sc site appeared to be due to in situ deiodination, rather than to transfer of the parent compound into the circulation. After T3 administration, both iv and sc, a product with kinetics similar to RISA appeared, accounting for 3% of the T3 decay for the averaged data and ranging from 0.9--19.4% in individual cases. Comparing T3 kinetic analysis by this technique (in which the iodoprotein byproduct is accounted for by modeling instead of chemical separation), the resulting parameters are similar to those reported by others after chemical separation of T3. Judging by compartment size and distributional kinetics, the model compartments derived for iv administration of these compounds appear to represent the vascular pool (central compartment), the hepatic and renal distribution sites (fast peripheral compartment), and other peripheral tissues (slow peripheral compartment). The latter, which presumably includes the site of sc injection, transfers into the central compartment at approximately the same rate as does the compartment representing the sc injection site itself.