Brand vs. Generic Levothyroxine: Effect on TSH Lab Data Compared
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Brand Synthroid (levothyroxine) was associated with significantly better thyroid stimulating hormone (TSH) lab results vs. generic levothyroxine, according to a retrospective claims analysis presented at the AACE 2017 Annual Meeting.
The bioequivalence of drugs with a narrow therapeutic index (eg, levothyroxine) has always been concerning. The literature comparing brand vs. generic levothyroxine have produced opposing findings and so study authors analyzed retrospective claims to explore TSH lab outcomes among patients with hypothyroidism treated with Synthroid vs. generic levothyroxine.
Claims from the Optum Clinformatics Data Mart database was used to identify patients with at least 1 hypothyroidism diagnosis code between January 2008 and March 2016 who were started on either Synthroid or generic levothyroxine within a year of being diagnosed. "Patients below 18 years old, with diagnosis of thyroid cancer, on combination T3/ T4, or with last lab data missing or invalid, were excluded," added Kathleen Wyne, MD, PhD, Ohio State University Wexner Medical Center.
Generic levothyroxine users (n=28,034) were matched 2:1 to brand Synthroid patients (n=14,017) based on age, sex, and region. The study's primary outcome was the proportion of patients for whom the last TSH lab outcome during the follow-up period was outside the reference range (<0.3mIU/L or >4.12mIU/L).
During follow-up, the final TSH lab outcomes were out of range for 22.6% of patients in the generic levothyroxine arm vs. 20.9% in the Synthroid arm (P<0.0001). For Synthroid users, the likelihood of having a TSH lab outcome out of range was 0.89 (95% CI: 0.85-0.94; P<0.0001) compared to generic levothyroxine users.
"In our study, we found that Synthroid was associated with an 11% lower likelihood of having TSH labs out of range when compared with similar patients taking generic levothyroxine," explained Dr. Wyne. Study confounders such as insurance type, Charlson comorbidity index score, year of index date, and type of diagnosing physician were accounted for, and the study cohort was "robust," but claims data carry limitations that should be considered when interpreting the results, the authors cautioned.
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