Ratio of serum triiodothyronine to thyroxine and the prognosis of triiodothyronine-predominant Graves' disease.

Triiodothyronine (T3)-predominant Graves' disease is characterized by persistently high serum T3 level, normal serum thyroxine (T4) level, and high (greater than 20) serum T3/T4 ratio (nanograms/micrograms) during thionamide drug therapy. We studied the clinical course of 30 patients with T3-predominant Graves' disease. After receiving drug therapy for 1 to 4 years, 27 patients with T3-predominant Graves' disease had relapses, whereas only 9 control patients with Graves' disease whose serum T3/T4 ratio had become persistently normal (less than 20) had relapses. The T3-predominant patients had greater serum TSH receptor antibody activity, thyroid T4 5'-deiodinase activity, and decreased T3 content of thyroglobulin when compared with the control patients. Our findings show that patients with T3-predominant Graves' disease are unlikely to have a long-term remission with drug therapy. The cause of high serum T3/T4 ratio is due, in part, to the more active thyroid T4 5'-deiodinase that may be mediated by high levels of Graves' immunoglobulin.