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Brand-name levothyroxine was no better than generic formulations at normalizing thyrotropin levels, a new study found.

Within 3 months of filling a levothyroxine prescription, 75.4% (95% CI 71.9%-79.0%) of patients on a generic formula achieved normal thyrotropin levels versus 76.9% (95% CI 73.4%-80.6%) of those on a brand-name formula (P=0.23), according to Juan Brito, MD, MSc, of the Mayo Clinic in Rochester, Minnesota, and colleagues.

After 3 months on the medication, there was also no significant difference in the proportion of patients who still had abnormal thyrotropin levels -- 4.1% of those on a generic versus 3.9% of those on a brand-name formulation (P=0.65).

Neither formulation outperformed the other in the maintenance of normalized thyrotropin levels, either (between 0.3 mIU/L to 4.4 mIU/L), the group reported in .

Among patients who achieved normal values within 3 months of starting levothyroxine, 82.6% of those on a generic formula maintained normal levels during the subsequent 3 months versus 83.8% of those on a brand-name formula (P=0.62).

"During the last decade, of all levothyroxine prescriptions filled were still for brand-name levothyroxine products," Brito's group pointed out. "Although the use of generic levothyroxine has increased over time in the United States, it continues to be relatively low compared with the use of other generic medications."

They explained that one reason why healthcare providers may still be opting for brand-name levothyroxine could stem from their skepticism about how effective the generic options are.

"After the FDA approved the use of several generic levothyroxine products, 3 endocrine professional societies released a in 2004 raising questions about the FDA's method for determining bioequivalence," they noted.

However in response to this, the FDA took extra measures to tighten potency specifications in 2007 in attempts to ameliorate concerns over the effectiveness of generics. As part of this effort, the stated that all levothyroxine sodium drug products meet 95% to 105% of potency specifications until their expiration dates.

In this retrospective analysis, a total of 17,598 adults had newly filled a prescription for generic or brand-name levothyroxine from 2008 to 2017. The vast majority of patients used generic formulations (87%), and the average dose was below 50 μg/d.

All individuals had baseline thyrotropin levels of 4.5 to 19.9 mIU/L and those with baseline levels exceeding 20.0 mIU/L were excluded since that extent of thyroid dysfunction would likely take longer than 3 months to stabilize.

Among those on a brand-name formulation, 89% were on Synthroid, followed by about 8% on Levoxyl. Generic manufacturers represented in the analysis included Mylan Pharmaceuticals, Sandoz AG, and Lannett Company.

Some limitations to the study included the observational nature of the analysis, as well as an inability to control for other factors that could influence absorption of the medication, like concomitant use of over-the-counter calcium.

"Further research needs to clarify whether these findings are consistent among patients with no or little endogenous thyroid hormone production and whether switching between formulations affects these outcomes," the group concluded.

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