The present study evaluates the sequential extrathyroidal monodeiodination of thyroid hormones through tri-, di-, and monoiodothyronines in chronic renal failure (CRF) in man. Simultaneous turnover studies of T4, T3, rT3, 3, 5-diiodothyronine (3, 5-T2), 3, 3′-T2, 3′, 5′-T2, and 3′-monoiodothyronine (3′-Ti) were conducted in six patients with CRF (creatinine clearance, 9-18 ml/min) using the single-injection, noncompartmental approach. Serum levels of T4, T3, and 3, 5-T2 were reduced to two thirds of control levels (P < 0.05), whereas serum rT3 and 3, 3′-T2 levels were reduced to a minor degree. Serum 3′, 5′-T2 levels were unaffected by CRF, but in contrast, serum 3′-Ti was doubled (P < 0.05). The MCRs of T4 rT3, and 3′, 5′-T2 were enhanced to 168%, 127%, and 187% of normal (P < 0.05), respectively, whereas those of T3, 3, 5-T2) 3, 3′-T2, and 3′-Ti were unaffected. The mean production rates (PRs) of the iodothyronines in CRF were as follows (CRF vs. control values, expressed as nanomoles per day/70 kg): T4, 119 vs. 125; T3, 26 vs. 44 (P < 0.01); rT3, 49 vs. 48; 3, 5-T2, 3.5 vs. 7.2 (P < 0.001); 3, 3′-T2, 25 vs. 35 (P < 0.01); 3′, 5′-T2, 25 vs. 14 (P < 0.01); and 3′-T, 39 vs. 30. Previous studies have demonstrated reduced phenolic ring (5′-) deiodination of T4 in CRF, which is supported by the present finding of unaltered PR of T4 and reduced PR of T3. In contrast the 5′-deiodination of T3 leading to the formation of 3, 5-T2 was found unaffected by CRF, since the conversion rate (CR) of T3 to 3, 5-T2 (PR 3, 5-T2/PR T3) was unaltered (16% vs. 15% in controls). The tyrosylic ring (5-) deiodination of T4 to rT3 was unaffected in patients with CRF, the CR being 42% vs. 40% in controls, in contrast to an enhanced CR of rT3 to 3′, 5′-T2 (53% vs. 29%, P < 0.01), which also is a 5-deiodination step. In conclusion, our data show that CRF profoundly changes the kinetics of all iodothyronines studied. Furthermore, our data are compatible with the existence of more than one 5′-deiodinase as well as more than one 5-deiodinase in man.