The aim of the present study was to evaluate the bioequivalence and therapeutic equivalence of the two most commonly prescribed L-thyroxine (monsodium L-thyroxine hydrate, CAS 25416-65-3) formulations in Brazil in patients treated for hypothyroidism. Twenty-four patients received 100 micrograms L-thyroxine daily of either Puran T4 (test) or the Brazilian reference formulation (reference) during 42 days, in a two-period crossover design. Serum samples obtained over a 24-h interval were analyzed for their total T4 concentration by a chemiluminescent immunoassay. Content and uniformity of the tablets and dissolution studies were also assessed according to USP 24 monograph using an isocratic HPLC-UV system and a rotating-paddle method. The mean pharmacokinetic parameters for total T4, expressed as geometric means (CV), for the test and reference were, respectively: Cmax (microgram/dl) 9.8 (14.3%) and 10.8 (14.9%); AUC0-24 h (microgram/dl.h) 206.8 (13.9%) and 230.4 (14.9%). Median values (90% CI) for Tmax (h) were 3 (2-3) and 2 (2-4) for the test and reference, respectively. 90% CI for ratios of LogCmax and LogAUC0-24 h were 86.6-94.9 and 86.3-93.4, respectively. Although the test exhibited values of Cmax and AUC0-24 h around 10% lower than the reference, these formulations must be considered bioequivalent since the 90% CI for both Cmax and AUC0-24 h mean ratio were within the 80-125% interval as proposed by the US Food and Drug Administration and the Brazilian legislation. TSH dosages within the normal range further support therapeutic equivalence between the two formulations. Dissolution data were roughly in agreement with in vivo results since both formulations comply with the USP dissolution criteria although the test tablets had a slower dissolution rate than the reference tablets. As a conclusion, the two oral formulations of L-thyroxine are both bioequivalent and therapeutically equivalent although presenting a small difference in their extent of absorption. Noteworthy, the dissolution profiles of the tablets correlate well with their bioavailability in the present experimental conditions.