A new method for measurement of the conversion ratio of thyroxine to triiodothyronine in euthyroid man

A new method for the measurement of the conversion ratio (CR) of T4 to T3 in euthyroid man is described. In contrast to previously described studies, this investigation relied on sampling of urine rather than plasma after isotopic labeling of the study subject. The CR value determined in six euthyroid men was 0.482 +/- 0.014. Thus, approximately half of the daily T4 production is converted to T3 as determined by this method. A major advantage of this technique is its reproducibility, as demonstrated by the low coefficient of variation of 6.8% in the study group, which is not significantly different from the 6.7% coefficient of variation for replicate determinations on the same sample. Thus, this method may be useful tool in comparing the CR of T4 to T3 in man under varying conditions even if only small differences in conversion efficiencies exist between groups. One apparent discrepancy observed in the present study is that the calculated T3 produced from the conversion of T4 exceeded the simultaneously calculated daily T3 production rate measured in blood. The cause of this discrepancy is presently unknown, but may represent T3 produced from T4 by renal or extrarenal sources which is excreted without contributing to the T3 blood production rate. However, the reproducibility of the method and the smaller amounts of labeled isotope required show promise that this technique may be useful in assessing T4 to T3 conversion in a variety of altered metabolic states.