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To the Editor.—
In a recent letter Jacobson and his colleagues (243:733, 1980), reported observations that led them to conclude that the bioavailability of thyroxine (T4) is significantly greater in Levothroid (formerly Letter) than in Synthroid, both commercial preparations of levothyroxine sodium. In ten patients who had diffuse goiter, hypothyroidism, or thyroid nodules, serum T4 concentrations after an initial month of treatment with 100 μg of Levothroid daily (14.1±1.0 μg/dL) were significantly greater than those seen after a subsequent month of comparable treatment with Synthroid (10.9±1.1 μg/ dL). Though numerical data were not presented, serum thyroid-stimulating hormone concentrations were reported to be generally lower during treatment with Levothroid than during treatment with Synthroid. The writers did not suggest that either preparation was preferable to the other, merely that they were different.We are writing now because our own experience in this regard has been different. In somewhat similar
Ingbar JC, Braverman LE, Ingbar SH. Equivalence of Thyroid Preparations. JAMA. 1980;244(10):1095. doi:10.1001/jama.1980.03310100015014