Here we report the case of a young girl who had vague signs and symptoms potentially attributable to hyperthyroidism and was found to have autoimmune thyroiditis and hyperthyroxinemia. The elevated serum free thyroxine levels were persistent when measured by both standard assays and equilibrium dialysis/high-pressure liquid chromatography-tandem mass spectrometry. The clinical symptoms, with discordant thyroid test results, created a diagnostic dilemma that led initially to unnecessary additional evaluations. She was ultimately found to have familial dysalbuminemic hyperthyroxinemia (FDH) and required no therapy. This case highlights the inherent difficulties in evaluating children, who typically have vague signs and symptoms of thyroid dysfunction, when, in addition, they have an unrelated acquired (autoimmune) as well as a genetic (FDH) defect. The benefit of including testing for immediate members of the family is emphasized.