Substitution of Levothyroxine Products

To the Editor.—  Since calling attention to the problem of generic substitution of levothyroxine products in a recent letter (240:1241, 1978), we recently encountered evidence demonstrating that in one case, a generic product was not equal in potency to the brand name product.A 68-year-old woman who had primary hypothyroidism and a large goiter returned for a routine examination. Instead of receiving the brand name levothyroxine product we prescribed, the pharmacist had switched the prescribed drug to a look-alike generic product. While she was receiving 0.1 mg daily of generic levothyroxine, the patient's free thyroxine index was 1.0 (normal, 1.4 to 4.0), and by radioimmunoassay, the serum thyroid-stimulating hormone level was 76.0 μunits/mL (normal, less than 10). The drug regimen was changed to 0.1 mg daily of the brand name levothyroxine product, and seven weeks later, the free thyroxine index was 2.1, and the serum thyroidstimulating hormone level was 8.6