Serum triiodothyronine concentration in the diagnosis of hyperthyroidism.

The serum triiodothyronine concentration is superior to the serum thyroxine concentration, the resin uptake test and the free thyroxine index in the diagnosis of hyperthyroidism. Over a 14 month period fifty-five patients attending an endocrine clinic with suspected thyrotoxicosis of all degrees of severity had blood taken on initial attendance and the serum was stored for routine thyroid function tests and triiodothyronine estimation. The patients were followed up and forty-six proved to be toxic and seven to be euthyroid; two could not be classified. Analysis of the initial serum showed that the serum triiodothyronine concentration was superior to the serum thyroxine concentration, the resin uptake test and the free thyroxine index in predicting the clinical outcome.