Euthyroid Patient with Elevated Serum Free Thyroxine George van der Watt, 1* David Haarburger, 1 and Peter Berman 1 CASE 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 immunosup- pression with prednisone and azathioprine and was cur- rently in remission. On examination, she was found to be clinically euthyroid without a goiter. She had also devel- oped a diffuse erythematous papular rash on the face and back, with bullous lesions on the chest. Immunofluores- cent 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 Cen- taur® Immunoassay System (Siemens Medical Solutions Diagnostics) revealed an increased concentration of se- rum free thyroxine (FT 4 ) 2 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 IU/mL) (reference range 0.35–5.5 mIU/ L), and normal free triiodothyronine (FT 3 ) 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 FT 4 to 125.3 pmol/L (9.7 ng/dL) and 155 pmol/L (12.0 ng/dL), respectively. TSH and FT 3 remained within reference intervals, as did total T 4 by RIA, at 155 nmol/L (12 g/dL) (reference range 58 – 161 nmol/L). Furthermore, she tested positive for anti- thyroperoxidase 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 dis- concordant thyroid function tests was initiated. DISCUSSION Increased serum FT 4 concentrations, with a nonsup- pressed TSH, are most often seen with erratic thyroxine replacement therapy. Other less common causes in- clude antibody interference in the FT 4 assay causing falsely increased FT 4 , thyroid hormone resistance syn- dromes, TSH-secreting pituitary adenoma, familial dys- albuminemic hyperthyroxinemia, amiodarone therapy, and primary hyperthyroidism with antibody interference in the TSH assay causing falsely increased TSH results. More recently, patients have been described with type 2 deiodinase deficiency due to defective selenoprotein syn- thesis; these patients presented with increased FT 4 and normal FT 3 and TSH (1 ). In this case, antithyroxine autoantibody interfer- ence in the FT 4 assay appeared likely, as the FT 3 was within reference limits, in keeping with a clinically eu- thyroid state. The same patient sample yielded euthy- roid FT 4 when measured using 2 other immunoassay platforms, namely 14.5 pmol/L and 13.4 pmol/L on the AxSYM and Architect platforms (Abbott Diagnostics), respectively. This discrepancy could be explained by the fact that the Advia Centaur design uses a 1-step analog-based FT 4 immunoassay. In this assay, the analog is a high-molecular-weight IgG-T 4 complex la- beled with acridinium ester (2). Serum and labeled FT 4 analog are introduced into the reaction cuvette si- multaneously, and sample FT 4 and analog T 4 compete for solid-phase antibody. Unbound material is then washed out, and the remaining bound analog T 4 is measured by chemiluminescence. Increased lumines- cence therefore relates to decreased FT 4 and vice versa. During this single incubation, anti-T 4 antibodies in the sample may bind the analog T 4 , preventing its binding to the solid-phase antibody, leading to reduced lu- minescence signal and a falsely increased FT 4 result. In contrast, both the Abbott AxSYM fluoroimmuno- assay and the Abbott Architect chemiluminescence immunoassay for FT 4 use a 2-step design in which labeled analog T 4 is introduced only after unbound material from the sample has been removed in a wash step, thereby precluding interaction between sample antibodies and hormone analog. The fact that there was not marked interference in the total T 4 assay in this case might be because FT 4 assays sequester a much smaller fraction of the total T 4 in serum to mini- 1 Division of Chemical Pathology, National Health Laboratory Service, Groote Schuur and Red Cross Children’s Hospitals, University of Cape Town, Observa- tory, South Africa. * Address correspondence to this author at: Division of Chemical Pathology, National Health Laboratory Service, Groote Schuur and Red Cross Children’s Hospitals, University of Cape Town, Observatory, South Africa, 7925. Fax +27216585225; e-mail george.vanderwatt@chempath.uct.ac.za. Received December 5, 2007; accepted April 4, 2008. Previously published online at DOI: 10.1373/clinchem.2007.101428 2 Nonstandard abbreviations: FT 4 , free thyroxine; TSH, thyroid-stimulating hor- mone; FT 3 , free triiodothyronine; THAAb, thyroid hormone autoantibody. Clinical Chemistry 54:7 1239–1247 (2008) Clinical Case Studies 1239