Experts warn on levothyroxine potency differences

15 August 2004

Two US organizations, the American Thyroid Association and the AmericanAssociation of Clinical Endocrinologists, representing more than 5,400 clinical endocrinologists, have warned patients taking thyroid medication, prescribing physicians and pharmacists dispensing these drugs that clinically-important differences do, in fact, exist between one recently-approved generic levothyroxine preparation and the most widely-prescribed brand.

Levothyroxine is taken by more than 13 million Americans to treat an underactive thyroid, thyroid gland enlargement, nodules or cancer. In June, the US Food and Drug Administration ruled that several generic levothyroxine preparations had the same clinical effect and safety profile as certain branded products. As a result, pharmacists may substitute a patient's current levothyroxine preparation for another, sometimes without their physician's approval.

According to bioequivalence data used to acquire FDA approval, one recently-approved generic levothyroxine preparation (Sandoz' Levothyroxine Sodium) is significantly more potent than the most widely-used brand of levothyroxine (Abbott Laboratories' Synthroid). Information from bioequivalence studies submitted to the FDA show that the new generic may be as much as one-eighth more potent (+12.5%) than the widely-prescribed branded product.

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