Abstract. The validity of estimation of the production rates of T3 and rT3 in man based on noncompartmental analysis of blood-derived data has been questioned owing to incomplete exchangeability of T3 and rT3 between plasma and extrathyroidal tissues in which a local production of these iodothyronines takes place. The possible existence of a nonexchangeable or hidden pool of T3 and rT3 would result in an underestimation of the daily production. By contrast, the production rate of T4 can be estimated reliably using noncompartmental analysis. We have studied 16 women with pretreatment severe hypothyroidism on constant levothyroxine therapy. Simultaneous measurements of T4, T3 and rT3 production rates were performed using bolus injection of radiolabelled iodothyronines. The tracers were isolated from plasma using gel separation/antibody extraction, and production rates were calculated by noncompartmental analysis. Mean (± sd) production rate of T4, T3 and rT3 were: 119 ± 43, 40.0 ± 22.0 and 54.9 ± 20.0 nmol · day−1 · (70 kg)−1, respectively. Thus 79.5 ± 7.0% of T4 was deiodinated into T3 and rT3. This leaves 20.5% to other metabolic pathways of T4 and to a possible underestimation of T3 and rT3 production rate. Based on conservative estimates from the literature, the other metabolic pathways of T4 amount: oxidative deamination 1.1%; ether link cleavage 0%; urinary excretion 2.5%; and fecal excretion 14%. Thus, the various metabolic pathways seem to explain 97% of daily produced and degradated T4 in man. Therefore the understimation of T3 and rT3 production rates in man using noncompartmental analysis seems of little if any importance, and existence of a hidden pool of these iodothyronines may be questioned.
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