To the Editor.—
The Coronary Drug Project's report (220:996, 1972) of long-term therapy with dextrothyroxine sodium refers to calorigenic effects due to levothyroxine sodium contamination as possible explanation for the adverse effects. This observation has led to the discontinuation of the drug regimen in the project.We have assayed several samples of the dextrothyroxine commercially available in the United States by a modification of a very sensitive method1 that allows detection of levothyroxine contamination as low as 0.1%. The dextrothyroxine preparations we tested were found to contain 0.8% to 1.0% of levothyroxine. Daily doses of 6 mg of dextrothyroxine sodium thus contain up to 60μg levothyroxine sodium, an amount that may very well exert calorigenic effects significant in regard to aggravation of coronary heart disease.