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A bioequivalência e biodisponibilidade das preparações comerciais de L-tiroxina podem apresentar diferenças significativas entre si. O objeto do presente trabalho foi comparar variáveis séricas tireóideas (T4 e T3 totais, T4 livre e TSH endógeno) em pacientes previamente tireoidectomizados (há mais de 5 anos) e sem evidência clínica ou ecográfica de tecido glandular cervical remanescente. Os referidos parâmetros foram medidos, por 15 dias, a intervalos de tempo regulares, durante administração de solução oral de L-tiroxina (100µg), presumivelmente de absorção e biodisponibilidade ideais. Comparou-se, em seguida, a oscilação das mesmas variáveis com duas outras preparações comerciais, sob forma de comprimidos de L-tiroxina (LT4A e LT4B) nas doses de 100µg/dia e a seguir 200µg/dia. Não se observou diferença significativa entre a solução oral e as duas preparações comerciais nos vários tempos de medida. Tampouco houve diferença significativa entre os comprimidos de L-T4A e L-T4B em doses de 100µg ou de 200µg/dia. Concluímos que ambas as preparações apresentam biodisponibilidade e bioequivalência adequadas nas condições utilizadas neste estudo. A major issue influencing prescribing patterns of physicians has been the relative bioavailability and bioequivalence of commercial levothyroxine preparations. Some studies have suggested similar bioequivalence between various L-T4 tablets while other reports have found differences. Some of these studies have been criticized for methodological defects in patients' selection or study design, which have become appreciated only in retrospect. Moreover the US Pharmacopeia has proposed that commercial tablets of L-T4 should be tested for its dissolution in two different solutions. Each manufacturer should periodically test batches of tablets as indicated. In the present study, we have compared serum thyroid function parameters (total T4, total T3, free T4, and serum TSH) in previously totally thyroidectomized patients, without sonographic evidence for reminiscent thyroid tissue in the cervical area. The above parameters were measured, as function of time, for 15 days of either an oral solution of L-T4 (100µg) or tablets of L-T4A and L-T4B (100µg/tablet). There was no significant difference between the various parameters, in relation to time, for each of the different preparations. Moreover L-T4A tablets did not differ from L-T4B tablets, when those parameters were tested, either at the concentration of 100 or 200µg/day. We conclude that the two oral formulations of L-thyroxine have similar bioavailability and bioequivalence under the present experimental conditions

Bioequivalência de Preparações Comerciais de L-Tiroxina (100 e 200µg): Avaliação em Pacientes Hipotireóideos Previamente Tireoidectomizados