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

Juan P. Brito, Yihong Deng, Joseph S. Ross, Nam Hee Choi, David J. Graham, Yandong Qiang, Elena Rantou, Zhong Wang, Liang Zhao, Nilay D. Shah, Kasia J. Lipska

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)349-358
Number of pages10
Issue number2
StatePublished - May 2022


  • Generic
  • Hypothyroidism
  • Levothyroxine

ASJC Scopus subject areas

  • Endocrinology, Diabetes and Metabolism
  • Endocrinology


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