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Furosemide is a commonly used diuretic available in both oral and intravenous formulations. Furosemide is a known inhibitor of thyroid hormone binding in serum (1), and high-dose treatment with furosemide can lower total thyroxine and increase its free fraction in vivo (2). The difficulty in assessing thyroid status in severe illness is well known (3), and this difficulty can be complicated by treatment with drugs such as furosemide. The following study was undertaken after investigation of a patient in the coronary care unit with a thyroid-stimulating hormone (TSH) concentration within the health-related reference interval coupled with a free thyroxine of >77 pmol/L on the Abbott AxSYM immunoassay analyzer (reference interval, 9–24 pmol/L). In a subsequent sample, the free thyroxine was 16.2 pmol/L. The error was attributed to contamination of the sample by intravenous furosemide. I investigated the effects of furosemide on the measurement of free thyroxine and TSH, using three different assay systems: the AxSYM from Abbott Laboratories, the ACS:180 from Chiron, and the Vitros from Ortho Clinical Diagnostics.
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Robert C. Hawkins (1998) studied this question.
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