Case report: over-substitution of thyroxine due to interference in serum thyroid-stimulating hormone measurement.

A 70-year-old female patient underwent thyroidectomy and thyroablative radioiodine treatment for folliculary thyroid cancer with pulmonary metastases (pT1, NO, M1). For thyroid hormone replacement therapy, serum thyroid-stimulating hormone (TSH) concentrations were aimed to be below the lower limit of quantification of the assay (-0.03 mU/mL) with concentrations of free thyroxine and free triiodothyronine within the reference range. For TSH measurements, the Elecsys immunoanalyzer system (Roche Diagnostics, Mannheim, Germany) was used in the routine endocrinological laboratory of our institute. TSH suppression is usually achieved with daily thyroxine doses of approximately 1.5 and 2.0 mg/kg. In the patient reported herein, the starting dose of 150 mg/day was repeatedly increased, as TSH was constantly found detectable. Under a dose of 300 mg thyroxine/day, a TSH concentration of 0.2 mU/mL was measured. At this time, free serum thyroxine was increased with 2.9 ng/dL (37.33 pmol/L) (reference range: 0.9–1.9 ng/dL (11.58–24.46 pmol/L) and the patient complained about symptoms consistent with thyrotoxicosis. The constellation of hyperthyroid symptoms along with increased thyroid hormone concentrations in the presence of TSH levels only slightly below the normal range was found implausible. Therefore, TSH was re-analyzed using a different immunoassay system, implemented in the short turnaround time laboratory of the institute (Centaur TSH assay, Bayer Diagnostics, Tarrytown, NY, USA). Using this assay, TSH levels were below the lower limit of quantification. Consequently, interference of TSH measurement was noted in this case, and the dosage