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Serum Triiodothyronine to Thyroxine Ratio: A Newly Recognized Predictor of the Outcome of Hyperthyroidism due to Graves' Disease^*

First Department of Internal Medicine, Osaka Medical College Takatsuki 569 Kuma Hospital, Kobe 652, Japan

Address requests for reprints to: Dr. Junta Takamatsu, First Department of Medicine, Osaka Medical College, 2–7, Daigakucho, Takatsukishi, Osaka 569, Japan.

Patients with untreated hyperthyroidism due to Graves' disease have a proportionally greater increase in the serum T₃ than in the T₄ concentration and, therefore, have an elevation of the serum T3 to T4 ratio. The aim of this study was to investigate the alterations of the serum T₃ to T₄ ratio in relation to the outcome of antithyroid drug therapy.

Of 47 patients with hyperthyroid Graves' disease, 37 patients had a serum T3 to T4 ratio greater than 20 ng/µg before therapy (normal range, 12–20; mean, 16.0). In 7 of 37 patients, serum T₃ to T₄ ratios remained highduring a 2-yr course of antithyroid drug therapy, and in 6 of them (86%), hyperthyroidism recurred after cessation of drug therapy. In the remaining 30 patients,the initial high serum T₃ to T₄ ratios decreased to normal (<20) during treatment, and 15 of them (50%) had a remission of the disease after cessation of the drug. Of the 10 patients with initial serum T₃ to T₄ ratios within the normal range, this ratio re mainednormal duringtreatment, and 8 (80%) had a remission. Goiter size was larger in patients with high serum T₃ to T₄ ratios, and a reduction of goiter size occurred in some patients (57%) with decreasing serum T₃ to T₄ ratios.

The serum T₃ to T₄ ratio is a simple and useful predictor of the outcome of antithyroid drug therapy in patients with Graves' disease. A ratio greater than 20 throughout therapy indicates that the likelihood ofrelapse is high, and a ratio below 20 either initially or during therapy is an indicator of prolonged remission.

^* This work was supported in part by USPHS Grant AM-15070.

Received August 8, 1985.

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