A feasible method for indirect quantification of L-T₄ in drugs by iodine determination

In this work, a method combining microwave-induced combustion (MIC) for sample preparation of commercial levothyroxine sodium (L-T₄) drugs (L-T₄: 25-200µg/tablet), and potentiometry with ion selective electrode (ISE) for iodine determination and subsequent indirect quantification of L-T₄ was proposed. The type and concentration of the absorbing solution were evaluated to select the most suitable conditions for this study. Using the MIC method, it was possible to use solutions as diluted as 150mmolL^-1 (NH₄)₂CO₃ (for samples containing 25-200µg of L-T₄/tablet) for I absorption. In these conditions, recoveries for L-T₄ were between 99% and 101%, and relative standard deviations were lower than 10%. The limit of detection for L-T₄ was 11.2µg/tablet, which is almost two times lower than the minimum concentration of L-T₄ in commercially available drugs. Thus, the MIC was suitable for the digestion of several L-T₄ drugs for subsequent I determination by ISE and indirect quantification of L-T₄. Furthermore, the proposed method presents high throughput with low reagent consumption and consequently lower waste generation, making it suitable for routine determination of L-T₄ in drugs. From the obtained results, it was possible to observe that one of the analyzed samples is not in agreement with the limits established by the United States Pharmacopeia, indicating the importance of the drug quality control. The United States Pharmacopeia establishes that each tablet must contain between 90% and 110% of the amount of active substance declared by the manufacturer.