Abstract. Peripheral immunoglobulin (Ig) G 3 -secreting cells and serum concentrations of interleukin (IL)-10, a classswitching factor to IgG 3 -secreting cells, increase in patients with intractable Graves' disease (GD). However, they are not practical for laboratory tests. To find more stable and easily detectable markers of disease intractability or disease severity in patients with GD or Hashimoto's disease (HD), we examined the serum concentration of IgG 3 in 58 euthyroid GD patients who had been undergoing antithyroid drug treatment for more than 5 years but still must continue drug treatment to maintain a euthyroid state (intractable GD), 26 GD patients who had maintained a euthyroid state for more than 2 years without any treatment (GD in remission), 20 untreated, thyrotoxic GD patients, 40 euthyroid HD patients treated with thyroxine (5 men and 35 women), 13 untreated, euthyroid HD patients, and 39 healthy volunteers. Serum concentrations of IgG 3 increased in euthyroid patients with intractable GD and in those with GD in remission, but serum concentrations of IgG were not altered. The ratio of serum concentrations of IgG 3 to total IgG (IgG 3 /IgG ratio) was higher in euthyroid patients with intractable GD than in those with GD in remission. Multiple logistic-regression analysis demonstrated that IgG 3 /IgG ratio and goiter size were independent factors in disease intractability of GD patients. These results suggest that IgG 3 /IgG ratio and goiter size may be used as independent markers associated with GD intractability.

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