Replacement Dose, Metabolism, and Bioavailability of Levothyroxine in the Treatment of Hypothyroidism

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Abstract
A change in the formulation of the levothyroxine preparation Synthroid (Flint) in 1982 prompted us to reevaluate the replacement dose of this drug in 19 patients with hypothyroidism. The dose was titrated monthly until thyrotropin levels became normal. The mean replacement dose (±SD) was 112±19 μg per day, significantly less (P<0.001) than the dose of an earlier formulation—169±66 μg per day — used in a similar study (Stock JM, et al. N Engl J Med 1974; 290:529–33). The fractional gastrointestinal absorption of a tablet of the current formulation is 81 percent, considerably higher than the earlier estimate of 48 percent. Using high-performance liquid Chromatographic analysis, we found that the current tablet contains the amount of thyroxine stated by the manufacturer. By measuring the bioavailability of the earlier type of tablet in five patients, we inferred that the strength of the previous tablet had been overestimated.