Case Report: Abnormal Thyroid Function Tests in a Patient and Two Normal Volunteers Treated with Salsalate

Abstract

There are only three prior reports of abnormal thyroid function tests in patients who have received salsalate, the salicylate ester of salicylic acid. The authors report an elderly clinically euthyroid man who had thyroid function tests suggestive of central hypothyroidism while taking salsalate but whose thyroid tests returned to normal after the drug was discontinued. They also studied thyroid function tests, including free thyroxine (FT4) and reverse (T3), in two normal volunteers who took salsalate 750 mg twice daily for 1 week. In the normal subjects, total T4 and FT4 began to fall within 24 hours after the first dose of salsalate, and remained suppressed for at least 24 hours after the drug was discontinued. This rapidity of effect by salsalate is previously undescribed. There was also a fall in FT4, probably due to the use of diluted serum in the equilibrium dialysis FT4 assay. Because FT4 measurement using diluted serum or equilibrium dialysis may cause falsely low FT4 measurements, the authors believe ultrafiltration may be the only reliable method of measuring FT4 in these patients.