Determination of Common Parameters Fo Iodothyronine Metabolism and Distribution in Man by Noncompartmental Analysis

A noncompartmental method is proposed for the calculation of the total distribution volume of exchangeable iodothyronine, the fractional irreversible removal rate from the exchangeable pool, and the metabolic clearance rate of labeled triiodothyronine (T3) and thyroxine (T4). The method depends on a graphic analysis and does not require the use of computers. The results of noncompartmental analysis of T3 and T4 distribution and metabolism are compared to single compartmental calculations in 12 subjects. Single compartmental analysis appeared to overestimate the distribution volume of T3 by a factor of 2.9, the metabolic clearance rate by a factor of 1.4, and underestimate the fractional removal rate by a factor of 0.48. Whereas single compartmental analysis did not introduce a significant error into the calculation of the metabolic clearance rate of T4, the distribution volume appeared to be overestimated by a factor of 1.4 and the fractional removal rate underestimated by a factor of 0.9.