Euthyroid Patient with Elevated Serum Free Thyroxine

Abstract

A 32-year-old female patient presented complaining of increased flushing, perspiration and heat intolerance of 3 months’ duration. Medical history included idiopathic thrombocytopenic purpura of 4 years’ duration, which had been treated by splenectomy after failed immunosuppression with prednisone and azathioprine and was currently in remission. On examination, she was found to be clinically euthyroid without a goiter. She had also developed a diffuse erythematous papular rash on the face and back, with bullous lesions on the chest. Immunofluorescent antibody studies performed on a punch biopsy of skin were positive for several autoantibodies, leading to a diagnosis of subacute cutaneous lupus erythematosus. This diagnosis was further characterized by positive titers of nuclear and double-stranded DNA autoantibodies in the serum. Thyroid function testing on an Advia Centaur® Immunoassay System (Siemens Medical Solutions Diagnostics) revealed an increased concentration of serum free thyroxine (FT4)1 of 90.1 pmol/L (6.97 ng/dL) (reference range 11.5–22.7 pmol/L), a nonsuppressed thyroid-stimulating hormone (TSH) concentration of 1.8 mIU/L (1.8 μIU/mL) (reference range 0.35–5.5 mIU/L), and normal free triiodothyronine (FT3) concentration of 4.2 pmol/L (0.33 ng/dL) (reference range 3.5–6.5 pmol/L). Repeat investigation 1 and 2 months later revealed a progressive increase in FT4 to 125.3 pmol/L (9.7 ng/dL) and ≥155 pmol/L (≥12.0 ng/dL), respectively. TSH and FT3 remained within reference intervals, as did total T4 by RIA, at 155 nmol/L (12 μg/dL) (reference range 58–161 nmol/L). Furthermore, she tested positive for antithyroperoxidase antibodies at 110 IU/L (reference range <37 IU/L) and antithyroglobulin antibodies at 149 IU/L (reference range <98.1 IU/L). An investigation of her disconcordant thyroid function tests was initiated. Increased serum FT4 concentrations, with a nonsuppressed TSH, are most often seen with erratic thyroxine replacement therapy. Other less common causes include antibody interference in the FT4 assay causing falsely increased …