Switching Between Levothyroxine Generics Does Not Alter TSH Levels

Results of a propensity-matched comparative effectiveness research study are providing insight into the effects of switching among different generic levothyroxine products in adult patients using generic levothyroxine.

Results of the study, which included data form more than 15,000 patients, suggests switching between levothyroxine generics was not associated with clinically significant changes in serum thyrotropin (TSH) levels.

“These results may reassure patients and clinicians that switching among generic levothyroxine products is unlikely to have substantial implications for treatment effects,” wrote investigators.

With disagreement between the FDA position that generic levothyroxine products are expected to be interchangeable without the need for TSH testing and the American Thyroid Association recommendation against switching between levothyroxine products sourced from different manufacturers, investigators sought to compare the effects of switching among generic levothyroxine products on TSH levels. To do so, investigators designed a comparative effectiveness research study with 1:1 propensity matching to compare TSH levels between patients identified from the OptumLabs Data Warehouse who continued taking the same sourced generic levothyroxine product and those who switched.

To identify a cohort for inclusion in their analyses, the investigators perfume da search of the OptumLabs Data Warehouse to identify adult patients aged 18 years or older who filled a generic levothyroxine prescription from January 2008-June 2019. To be considered for inclusion, patients needed to have had a stable drug dose, the same drug manufacturer, and a normal TSH level for at least 3 months before either continuing to take the same product or switching among generic levothyroxine products. Investigators defined normal TSH levels as 0.3-4.4 mIU/L.

Exclusion criteria for the analyses included being pregnant, a diagnosis of hypopituitarism or hyperthyroidism, or if they had a medical condition or were using medications that might affect thyrotropin levels. Individuals were also excluded if they filled prescription for other forms of thyroid replacement therapy between 6 months before then index date and when the first TSH level was obtained 6 weeks to 12 months following the index date.

The primary outcomes of interest for the study were the proportion of individuals with a normal or markedly abnormal TSH level, which was defined as less than 0.1 or more than 10.0 mIU/L, using the first available laboratory results obtained 6 weeks to 12 months following the index date. Investigators pointed out a propensity score model was developed to limit confounding when using logistic regression. Additionally, investigators also pointed out demographic information, comorbidities, and baseline TSH level were used as covariates in their analyses.

A total of 15,829 patients who filled a generic levothyroxine prescription were identified for inclusion in the investigators’ analysis. This cohort had a mean age of 58.9 (SD, 14.6) years, 73.4% were women, 71.4% were non-Hispanic White individuals, and 56.3% were received a daily dose of 50 μg or less. Of the 15,829 included in the study, 13,049 continued taking the same sourced preparation and 2780 switched generic levothyroxine preparations. From these, investigators created 2780 propensity-matched patient pairs. Among these pairs, the proportion of patients with a normal TSH level after the index date was 82.7% among nonswitchers and 84.5% among those who switched (Risk difference, -0.018 [95% CI, -0.038 to 0.002]; P=.07).

Further analysis suggested the proportion of patients with markedly abnormal TSH levels after the index date was 3.1% among nonswitchers and 2.5% among those who switched (risk difference, 0.007 [95% CI,-0.002 to 0.015]; P=.14). The mean TSH levels after index date among nonswitchers was 2.7 (SD, 2.3) mIU/L and 2.7 (SD, 3.3) among those who switched (P=.94).

“This comparative effectiveness research study suggests that switching among generic levothyroxine products sourced from different manufacturers was not associated with differences in achieving a normal TSH level or with changes in TSH levels compared with continuing to take the same sourced levothyroxine,” investigators added.

This study, “Association Between Generic-to-Generic Levothyroxine Switching and Thyrotropin Levels Among US Adults,” was published in JAMA Internal Medicine.