Two organizations representing more than 5,400 clinical endocrinologists today 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 of levothyroxine.
The members of these organizations, the American Thyroid Association and the American Association of Clinical Endocrinologists, specialize in treatment of hormonal disorders.
Levothyroxine is taken by more than 13 million Americans to treat an underactive thyroid, thyroid gland enlargement, nodules, or cancer. In June, the Food and Drug Administration (FDA) 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 (Synthroid(R)). 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.
Furthermore, levothyroxine is a drug known to have a narrow toxic-to- therapeutic ratio with significant clinical consequences of even minor excessive or inadequate dosing. Potential adverse events include symptoms, osteoporosis, atrial fibrillation, worsening of heart disease, preterm delivery in pregnancy, impaired fetal brain development, and high cholesterol.