Rates of, and factors associated with, switching among generic levothyroxine preparations in commercially insured American adults

Importance: Some practice guidelines warn against generic L-thyroxine preparation switching. Objective: To examine the rates of generic L-thyroxine preparation switching within one year of initiating L-thyroxine, and to examine factors associated with switching. Design and setting: Retrospective study using national data from a large administrative claims database from January 2008 through November 2018. Patients: Medicare or commercially insured adults (≥18 years) who filled a generic L-thyroxine preparation. Main outcome measures: At least one switch from one generic L-thyroxine preparation to another within 1 year of L-thyroxine initiation defined by prescription fills. Results: From January 2008 to November 2018, we included 483,390 patients who initiated generic L-thyroxine: mean (SD) age was 61.4 years (15.2), 75.2% were female, 72.6% were white. Within 1 year of initiating therapy, 98,013 (20%) switched to another L-thyroxine generic preparation at least once. In a multivariate logistic regression analysis, factors associated with switching included the number of pharmacies visited to fill L-thyroxine (>2 vs 1 adjusted OR [aOR] 7.15, 95% confidence interval [CI] 6.97–7.34), age ≥75 vs. <45 years (aOR 1.29, 95% CI 1.26–1.33), history of thyroid surgery (aOR 1.22, 95% CI 1.13–1.31), and first L-thyroxine fill date in 2018 vs. 2008 (aOR 3.32, 95% CI 3.14–3.51). Conclusions and relevance: One in five patients switched among generic L-thyroxine manufacturers within one year of treatment initiation. Generic L-thyroxine switching occurred more often when more pharmacies were used to fill L-thyroxine. Given existing guideline recommendations, additional studies should clarify the impact of generic L-thyroxine switching on thyroid hormone values.