Whole-Body Counting of131I-Labeled Triiodothyronine

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Abstract
The absorption and degradation rates of Il31-labeled triiodothyronine (T3) were measured in human subjects by the whole-body counting methods previously described for I131-thyroxine (T4). Results obtained in 18 euthyroid subjects were as follows: (a) There was a significant difference in the ratio of 0- and 4-hr counting sensitivity of T3 as compared with T4, suggesting significantly different spatial redistribution of T3 and T4 in the whole body during this time, (b) The variation in T3 counting sensitivity related to body weight was similar to that found with T4. (c) An average of 85% of an oral dose of T3 was found to be absorbed, (d) The T3 degradation rate was similar after oral and intravenous administration. However, after oral administration a significant time delay was observed before apparent degradation of T3 began. This has been attributed to temporary hepatic sequestration of the oral dose, (e) Fecal excretion of the administered T3 label (mean 10.4%) and fecal lag time (1.3 days) were similar to values observed using T4. (f) The mean degradation rate of T3 (b day-1) after intravenous administration was 0.53 as compared with a value of 0.11 for T4. The apparent mean volume of distribution of T3 was 31 1 compared with a mean volume of 9.5 1 for T4. The possible significance of these values in terms of the reported ratio of daily thyroxine iodine secretion to daily thyroxine iodine degradation (TSR/TDR) of 1.6 is discussed. It is concluded that the whole-body counting technique, when applied to T3 studies, is subject to some variations arising from erratic stool excretion, which can become an important factor for materials with a rapid biological rate of turnover.