L-thyroxine dosage: a reevaluation of therapy with contemporary preparations

Traditional L-thyroxine dosing formulas may overestimate the thyroid hormone requirement in patients treated with contemporary L-thyroxine preparations. We did clinical and laboratory assessments of 41 patients treated in successive periods with Levothroid (Armour Pharmaceuticals, Kankakee, Illinois) and Synthroid (Flint Division, Travenol Laboratories, Morton Grove, Illinois), obtaining 87 sets of data. Clinical subgroups were defined on the basis of the thyrotrophin response to thyrotrophin-releasing hormone. Normal responses were seen in 9 of 14 (64%) patients taking 100 micrograms/d and were associated with an average replacement dosage of 127 micrograms/d (1.7 micrograms/kg body weight). Nine of twenty-eight (32%) serum thyroxine values in the "physiologically replaced" group were elevated. Forty-three of fifty-four (80%) patients ingesting 125 micrograms or more had blunted responses (thyroxine "overreplaced"), averaging a daily dosage of 154 micrograms (2.14 micrograms/kg X d). No significant difference was found between Levothroid and Synthroid in predicting clinical group assignment. Guidelines for currently available L-thyroxine preparations should be revised and the recommended dosage reduced.