Figure 5.

Experimental paradigms to test whether 3,5-T2 is formed from T4/T3 in humans. (A) Twelve euthyroid healthy volunteers received a single oral dose of L-T3 (50 μg) and blood samples were drawn during the following 72 h (53). (B) Eighteen hypothyroid patients were switched from T4 to daily doses of 15 μg Liothyronine (T3) at week 1. Their daily T3 dose was then increased to 30 μg after week 2 and increased as necessary up to 60 μg T3 to maintain a normal serum TSH. Serum samples were drawn weekly and also for a kinetic study during the 8 h after the final T3 dose was administered in week 6 (54). (C) Serum concentrations (relative to individual concentrations at t = 0 h) for the two kinetic studies (see above) administering Liothyronine (T3) to euthyroid volunteers (upper panel) and hypothyroid patients (lower panel). Individual serum concentration data are provided in Supplementary Data Sheet 1. No clear increase was found in serum 3.5-T2 concentration in both paradigms and also no increase was observed for 3-T1AM, the postulated product generated from 3,5-T2 via combined deiodination and decarboxylation).