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Switching between generic levothyroxine products won't make much of a clinical difference, a new study suggested.

In a comparative effectiveness research study of 15,829 patients, those who switched up their generic levothyroxine product didn't see any change in serum thyrotropin (TSH) levels compared with those who continued taking the same product, Juan Brito, MD, MSc, of the Mayo Clinic in Rochester, Minnesota, and colleagues reported.

Comparing lab results 6 weeks to 12 months after the index date, the proportion with normal TSH levels (0.3-4.4 mIU/L) was no different between the group who switched their levothyroxine product and so-called non-switchers (84.5% vs 82.7%; P=0.07), according to the findings in .

Similarly, 3.1% of non-switchers had markedly abnormal TSH levels (<0.1 mIU/L or >10 mIU/L) versus 2.5% of switchers (P=0.14). And there were also no between-group differences in the proportion with abnormal TSH levels. Overall, average TSH levels were the same, at 2.7 mIU/L for both groups.

"Generic levothyroxine preparations are less expensive and have been rated by the FDA as bioequivalent to their brand name reference-listed drugs," Brito's group explained. "However, generic levothyroxine has been less widely prescribed than other generic pharmaceutical products."

"One potential reason for this may be the lingering concerns about an association between switching levothyroxine products sourced from different manufacturers and the stability of thyroid hormone levels," they continued.

"When clinicians prescribe a particular brand name product (indicating dispense as written), that specific product is dispensed to a patient for the duration of the prescription," the researchers pointed out. "In contrast, when clinicians prescribe generic levothyroxine, pharmacists can substitute a variety of generic levothyroxine preparations made by different manufacturers (e.g., Mylan, Sandoz, or Lannett) without requiring clinician notification or approval."

And this sparked concerns in the field, they noted, to the degree that the specifically recommended against levothyroxine switching from different manufacturers due to concerns of bioequivalence.

"Given these concerns, it is important to understand the implications of switching among different generic levothyroxine products for TSH levels," Brito's group wrote, explaining the impetus for the study.

For their analysis, the researchers drew upon data from deidentified administrative claims linked to lab results from commercially insured adults and Medicare Advantage enrollees. All patients included had filled a generic levothyroxine preparations from the top three most common manufacturers -- Mylan, Sandoz, and Lannett -- from 2008 to 2019. This included both new and prevalent users.

Roughly three-fourths of the study population were women, and the average age was 59. Over half of the cohort were receiving a daily levothyroxine dose of 50 μg or less. Exclusion criteria included pregnancy, hypopituitarism, hyperthyroidism, or any medical condition that can affect TSH levels.

During the analysis, 13,049 patients (82.4%) continued taking the same sourced preparation, while 2,780 patients (17.6%) switched between generic levothyroxine preparations. Using 1:1 propensity matching, the group compared 2,780 patient pairs to measure the implications of switching products.

Acknowledging that these findings conflict with current clinical practice guidelines, Brito's group said this should only reassure both patients and clinicians that switching among products is unlikely to bear any substantial impact.

They also suggested that these findings may help "curtail the relative underuse of generic levothyroxine in the U.S. because one of the main concerns of prescribing generic levothyroxine is the lack of control over switching among different generic products."

They noted that between 2007 to 2016, roughly three-fourths of thyroid hormone prescriptions were for generic levothyroxine versus 97% of other drugs that had both brand-name and generic options available.

"The underuse of generic levothyroxine has considerable financial implications for patients and health care systems," the group underscored.

One limitation to the study was the lack of a comparison group that specifically measured switching from brand-name levothyroxine products to generics.

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