A family with familial dysalbuminemic hyperthyroxinemia is described. The syndrome is inherited as an autosomal dominant trait and is characterized by marked elevation of serum thyroxine, due to increased binding of thyroxine to albumin, whereas serum triiodothyronine is normal. Serum free thyroxine is normal when measured with ultrafiltration or equilibrium dialysis, but artefactually high when measured with an analogue assay. The importance of the condition, which is harmless, lies in the misinterpretation of values with subsequent erroneous treatment of thyrotoxicosis. By using an ultrasensitive TSH method it is possible to discriminate between euthyroid and hyperthyroid patients and thereby to avoid incorrect diagnosis in subjects with euthyroid hyperthyroxinemia.