Authors’ Reply to Castello-Bridoux et al.: “Comment on Levothyrox^® New and Old Formulations: Are they Switchable for Millions of Patients?”

Excerpt

We are pleased to respond to the letter of Munafo et al. [1] as follows: in our levothyroxine article [2], we concluded that “a statistical analysis conducted in the conceptual framework of individual bioequivalence would have enabled: (i) documentation of possible higher intra-individual variability for the new compared to the old formulation and, hence, possible reconsideration of development of this new formulation” and “(ii) consideration of a possible subject-by-formulation interaction, allowing both regulatory authorities and prescribing clinicians to be better placed to manage and systematically supervise all patients during transition from the old to the new formulation”. We are of the firm opinion that a fundamental responsibility of a drug company is to determine and report these two sources of biological variability, when, as is the case for levothyroxine (T4) [a drug having a Narrow Therapeutic Index (NTI)], a new formulation is imposed on millions of patients. …