Value of routine serum-triiodothyronine estimation in diagnosis of thyrotoxicosis.

Thyroid-stimulating hormone (T.S.H.), triiodothyronine (T3), and thyroxine (T4) concentrations were estimated on 432 blood specimens from patients with a provisional diagnosis of thyrotoxicosis. 59 patients had a raised serum T3 concentration with T.S.H. and T4 levels in the normal range. Further information was obtained in 41 of these patients, and T3 toxicosis was diagnosed in 17 cases, thus providing a projected total of 24 patients with T3 toxicosis. In addition, there were 56 patients, with more common biochemical features of thyrotoxicosis, having both a raised serum T3 and T4 concentration. Therefore 30% of the patients considered thyrotoxic in this retrospective survey were only diagnosed with the aid of a serum-T3 estimation.