Evidence for Dual Peripheral 5’Deiodinases (5’D) Regulating Circulating T3 Levels in Man
The majority of circulating T3 in euthyroid man appears to be derived from conversion of T4 to T3 via 5’deiodinase (5’D) systems located in peripheral tissues. Recently, we have reported that a progressive reduction in the efficiency of T4 to T3 conversion occurs when circulating T4 levels rise from subnormal to supraphysiological concentrations (1,2). As shown in Fig. 1 (left panel), the greatest change in conversion efficiency of T4 to T3 is seen over the span of T4 values ranging between 0 and 5 µg/dl in eu-TBG-emic subjects. These data were obtained in 254 patients who had been placed on varying doses of chronic T4 therapy. Some of the T3 values in patients with identical serum T4 levels have been averaged in order to obtain a better overview of the mean pattern of the T3 response in the study population. In separate T4 tracer kinetic studies, we have found that a 50% conversion efficiency of T4 to T3 occurs at low serum T4 values which progressively drops to less than 15% at T4 levels in excess of 20 µg/dl (3).It is of interest that no apparent alteration in the efficiency of T4 to rT3 was detected over this same range of serum T4 values (2).
KeywordsSupraphysiological Concentration Apparent Alteration Southern California School Hanes Plot Half Maximal Velocity
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