Human Thyroxine Absorption: Age Effects and Methodological Analyses

Abstract

Experience with the use of simultaneous oral and iv radioiodine tracers of thyroxine (T₄) to measure T₄ absorption was reviewed to determine the effect of advanced age on T₄ absorption and to search for acceptable simplifications in the measurement methodology. (1) Age effect: In control subjects, T4 absorption did not differ with age between 21 and 69 years; it averaged 62.8 ± 13.5% (SD) in subjects over age 70 compared with 69.3 ± 11.9% in subjects aged 21-69 (p < 0.001). We conclude that, because of this small but significant reduction in T₄ absorption efficiency in the elderly, it is especially important to monitor individual response to replacement T₄ doses in this age group. (2) Methodology studies: Intrasubject coefficient of variation for T₄ absorption in triplicate studies averaged 8.9%, when an identical method was used at weekly intervals (n = 3). When the 3 studies were done by 3 separate methods (sequential po and iv ¹²³I-labeled T₄, sequential ¹²⁵I-labeled T₄ po and iv, and simultaneous po and iv doses of different tracers), the intrasubject coefficient of variation averaged 9.8% (_n = 8). When the absorption calculation used only one serum sample, taken after isotopic equilibrium was achieved at 24 h [the double isotope (eq) method], calculated T₄ absorption differed little from that calculated in the same subjects using a full serum time-activity curve and a noncompartmental analysis [the double isotope (AUC) method]. Compared with a compartmental model solution, both methods slightly overestimated T4 absorption. In 4 subjects given iv radioiodide as a third tracer at the same time as the oral and iv T₄ tracers, iodide kinetics were modeled together with T₄ kinetics. Projections from this set of models demonstrated that, as long as radioiodide contamination of the T₄ tracers is measured and accounted for in the standards, contaminating radioiodide has no clinically significant effect on T₄ absorption measurement. When an oral tracer of T₄ was considered alone, without an iv tracer to correct for T₄ metabolism, the estimates of T₄ absorption correlated in a curvilinear fashion with T₄ absorption as measured by the double isotope method. Except at very low absorption levels, the oral tracer alone is a poor predictor of T₄ absorption. We conclude that, when using a double isotope method to measure T₄ absorption, it is acceptable to rely on a single serum sample taken at 24 h, a time when isotopic equilibrium between the two tracers has been reached. As long as the administered tracers have been chromatographed to quantify iodide contamination and to confirm the absence of other contaminants, it is unnecessary to chromatograph the serum or to prepurify the tracers to remove radioiodide. This is because the iodide content has very little effect on the T4 absorption fraction when it is measured at 24 h. When only a single T₄ tracer is available, projections from an oral tracer dose alone are unreliable. In this situation, we recommend the use of separate studies, with oral and iv doses of the same tracer. A suggested protocol for carrying out this simplified method is provided.