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 FT4, thyroid hormone resistance syndromes, TSH-secreting pituitary adenoma, familial dysalbuminemic 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 synthesis; these patients presented with increased FT4 and normal FT3and TSH