• Guillermo Di Girolamo, Guillermo A Keller, Antonio R de Los Santos, Daniel Schere, and Claudio D Gonzalez.
• Clinical Pharmacokinetic Unit, Department of Pharmacology, School of Medicine, Universidad de Buenos Aires, Ciudad Autónoma de Buenos Aires, Argentina. gdigirolamo@fmed.uba.ar
• Clin Ther. 2008 Nov 1; 30 (11): 2015-23.

BackgroundLevothyroxine has a narrow therapeutic index; therefore, precise and accurate assessment of the bioequivalence of different levothyroxine products is critical. Bioavailability estimates of levothyroxine formulations might be affected by baseline concentrations of the hormone.ObjectivesThe aim of this study was to assess the bioequivalence of 100 microg of a test (T4 Montpellier 100, Química Montpellier S.A., Buenos Aires, Argentina) and reference (Synthroid, Abbott Laboratories, Abbott Park, Illinois) formulation of levothyroxine. We also compared 2 methods of levothyroxine measurements: without and with baseline correction for endogenous levothyroxine.MethodsThis randomized, open-label, 2-sequence, crossover study with a 65-day washout period was carried out in healthy, white, euthyroid volunteers following a single dose of sodium levothyroxine 600 microg. Blood samples were collected at 30 and 15 minutes prior to administration, and 0 (baseline), 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 12, 24, and 48 hours to determine thyroxine; serum thyrotropin (TSH) concentrations were determined 30 minutes before administration and 48 hours after administration. Serum concentrations of thyroxine were determined through radioimmunoassay and serum TSH concentrations were determined by a validated 2-site immunoradiometric assay. The formulations are considered to be equivalent if the 90% CI ratios for C(max) and AUC(0-last) are within 80% to 125%, per the US Food and Drug Administration (FDA). Adverse event monitoring was performed throughout the study by assessing clinical parameters (eg, blood pressure, electrocardiogram) and patient reports.ResultsA total of 24 volunteers (16 male, 8 female; mean [SD] age, 30.2 [4.6] years [range, 21-40 years]; mean [SD] weight, 71.71 [7.52] kg [range, 58-83 kg]) were included in the study. Without adjustment for baseline levels of endogenous levothyroxine, geometric mean C(max) for the test and reference formulations were 8.92 and 9.39 microg/dL, respectively; AUC(0-last) values were 368.40 and 383.37 microg/mL . h(-1). The 90% CI of the geometric mean for the percent ratios (test: reference) of C(max) and AUC(0-last) were 95.1% (90% CI, 91.9-98.3) and 96.1% (90% CI, 94.0-98.2), respectively. With adjustment for baseline levels of endogenous levothyroxine, the geometric mean C(max) for the test and reference formulations were 3.16 and 3.39 microg/dL, respectively; AUC(0-last) values were 88.33 and 95.60 microg/mL . h(-1). Despite performing the adjustment, the 90% CI of the geometric mean for Cmax and AUC(0-last) test:reference ratios were 93.1% (90% CI, 84.9-102.2) and 92.4% (90% CI, 85.2-100.2), respectively. No significant between-group differences were found with regard to pharmacokinetic parameters. No adverse events were observed or reported.ConclusionThe results of this study suggest that the test formulation was bioequivalent to the reference formulation of levothyroxine in these healthy volunteers, according to the US FDA definition of bioequivalence. This was supported by the analysis of concentration-time profiles without and with correction for basal endogenous levothyroxine.

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