Two cases of therapeutic failure associated with levothyroxine brand interchange

OBJECTIVE: To report the loss of therapeutic control in 2 hypothyroid patients and remind clinical pharmacists and other healthcare professionals to remain cognizant of possible product quality differences within or bioequivalency differences between levothyroxine products. CASE SUMMARIES: Two patients with stable hypothyroidism experienced symptoms of hypothyroidism with increased serum thyroid-stimulating hormone (TSH) concentrations after switching from 1 levothyroxine product to another. One tablet from 1 of the patient's levothyroxine prescriptions was assayed, and its levothyroxine content was 74.5% of the label claim, a value outside of the United States Pharmacopeia requirements of 90-110%. DISCUSSION: Two patients with hypothyroidism had remained euthyroid and stable while receiving 1 levothyroxine product, but became symptomatic with dramatically increased serum TSH concentrations while receiving what were thought to be comparable dosages of another levothyroxine product. Therapeutic control was reestablished in both patients after therapy with the original levothyroxine product was reinstated. CONCLUSIONS: Clinical pharmacists and other healthcare professionals should remain cognizant of possible product quality differences within or bioequivalency differences between levothyroxine products. These differences necessitate close monitoring of hypothyroid patients, counseling these patients about the clinical signs of sub- and supratherapeutic levothyroxine dosages, and prudence when switching patients with stable hypothyroidism to alternative levothyroxine products. If there are inconsistencies between levothyroxine products, resultant deleterious effects on the therapeutic stability of patients with hypothyroidism may undermine cost savings that might be incurred by such a change. If these patients are switched to alternative levothyroxine products, it is recommended that thyroid function tests be repeated after equilibration to the new product.