Usefulness of the 2nd generation assay for anti-TSH receptor antibodies to differentiate relapse of Graves' thyrotoxicosis from development of painless thyroiditis after antithyroid drug treatment for Graves' disease.

After antithyroid drug (ATD) treatment for Graves' disease, either a relapse of Graves' thyrotoxicosis or painless thyroiditis can develop. It is important to differentiate these two types of thyrotoxicosis because of the difference in required therapy. However, differentiation of thyrotoxicosis is usually difficult without radioactive iodine uptake (RAIU) which is not available in general practice. We investigated the clinical usefulness of the 2nd generation assay for anti-TSH receptor antibodies (TRAb) to differentiate these two types of thyrotoxicosis after ATD treatment for Graves' disease. We recruited 26 patients who developed thyrotoxicosis after ATD treatment for Graves' disease. These patients once became negative for TRAb and seemed to be in remission after ATD treatment. Upon development of thyrotoxicosis after ATD treatment, TSH, free T4, free T3 and TRAb were measured. TRAb were measured by the 2nd generation assay using recombinant human TSH receptors instead of porcine TSH receptors. Fourteen patients relapsed into Graves' thyrotoxicosis and 12 patients developed painless thyroiditis. Twelve (85.7%) of 14 patients with relapse of Graves' thyrotoxicosis were positive for TRAb. Eleven (91.7%) of 12 patients with development of painless thyroiditis after ATD treatment for Graves' disease were negative for TRAb. Levels of TRAb were significantly different between patients with relapse of Graves' thyrotoxicosis (4.86 +/- 6.45 IU/L) and those with painless thyroiditis (0.62 +/- 0.61 IU/L) (P<0.001). The 2nd generation assay for TRAb was useful to differentiate relapse of Graves' thyrotoxicosis from development of painless thyroiditis in patients who seemed to be in remission after ATD treatment for Graves' disease.