Ratio of Serum Triiodothyronine to Thyroxine and the Prognosis of Triiodothyronine-Predominant Graves' Disease

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Abstract
Triiodothyronine (T3)-predominant Graves'' disease is characterized by persistently high serum T3 level, normal serum thyroxine (T4) level and high (> 20) serum T3/T4 ratio (ng/.mu.g) during thionamide drug therapy. The clinical course of 30 patients with T3-predominant Graves'' disease was studied. After receiving drug therapy for 1 to 4 yr, 24 patients with T3-predominant Graves'' disease had relpases while only 9 control patients with Graves''s disease whose serum T3/T4 ratio had become persistently normal (< 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. Thus, patients with T3-predominant Graves'' disease are unlikely to have along-term remission with drug therapy. The cause of high serum T3/T4 ratios is due in part to the more active thyroid T4 5-deiodinase that may bedmediated by high levels of Graves'' Ig.