Detection of familial dysalbuminaemic hyperthyroxinaemia.

A simple test of in vitro thyroxine binding to serum proteins was used to screen serum samples from euthyroid patients with unexplained increases in the free thyroxine index. A diagnosis of familial dysalbuminaemic hyperthyroxinaemia was presumed in 14 unrelated subjects and six first degree relatives. Increased binding of thyroxine to thyroxine binding prealbumin was diagnosed in one woman with four unaffected relatives. Seven patients with familial dysalbuminaemic hyperthyroxinaemia had been treated for presumed thyrotoxicosis: two had typical Graves' disease and one subacute thyroiditis. Four other patients had been mistakenly treated with radioactive iodine or antithyroid drugs. In previously treated patients familial dysalbuminaemic hyperthyroxinaemia was suspected from the combination of a high serum thyroid stimulating hormone concentration and a normal but invalid free thyroxine index. Physicians should be cautious in accepting a diagnosis of thyrotoxicosis based mainly on a raised serum thyroxine concentration.