Clinical Pharmacokinetics

, Volume 58, Issue 7, pp 965–966 | Cite as

Comment on: “Levothyrox® New and Old Formulations: Are they Switchable for Millions of Patients?”

  • Joël CosteEmail author
  • Xavier Bertagna
  • Mahmoud Zureik
Letter to the Editor

In a recent “Current Opinion” in Clinical Pharmacokinetics, Concordet et al. [1] provided an analysis of the problem of symptoms reported by several thousands of patients in France after taking the new formulation of levothyroxine, mainly marketed as Levothyrox®. They claim that the problem can be solved using the conceptual framework of “individual bioequivalence” and performed a reanalysis of the data of the 204 healthy volunteers enrolled in the bioequivalence trial that was used to confirm the bioequivalence of the new formulation. This analysis, the results of which are given in Table 1 and Figure 1, involves the computation of two individual exposure ratios (IERs), one “unadjusted” using standard area under the curves (AUCs) of T4 concentrations measured at several times with new and old formulations (unadjusted IER = \(\frac{{\text{AUCnew}}}{{\text{AUCold}}}\)


Compliance with Ethical Standards


No external funding was used in the preparation of this letter.

Conflict of interest

Joël Coste, Xavier Bertagna, and Mahmoud Zureik have no conflicts of interest that are directly relevant to the content of this letter.


  1. 1.
    Concordet D, Gandia P, Montastruc JL, Bousquet-Mélou A, Lees P, Ferran A, et al. Levothyrox® new and old formulations: are they switchable for millions of patients? Clin Pharmacokinet. 2019. Scholar
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    Cox C. Delta method. In: Gail MH, Benichou J, editors. Encyclopedia of biostatistics. New York: Wiley; 1998. p. 1125–7.Google Scholar
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    Les jours. “Levothyrox: une étude confirme le laxisme de Merck et de l’ANSM”. 2019. Accessed 13 Apr 2019.
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    Le Monde. “Levothyrox: l’étude qui donne raison aux patients”. 2019. Accessed 13 Apr 2019.
  5. 5., “Scandale du Levothyrox: l’étude qui donne raison aux patients”. 2019. Accessed 13 Apr 2019.

Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Biostatistics and Epidemiology UnitHôpital CochinParisFrance
  2. 2.Paris Descartes School of MedicineParis Descartes UniversityParisFrance
  3. 3.EPI-PHARE (GIS ANSM-CNAM)ParisFrance
  4. 4.Versailles Saint-Quentin UniversityVersaillesFrance

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