Evaluation of serum basal thyrotrophin levels and thyrotrophin receptor antibody activities as prognostic markers for discontinuation of antithyroid drug treatment in patients with Graves' disease

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Abstract
We evaluated whether antithyroid drug treatment could be terminated more appropriately when both the serum basal thyrotrophin (TSH) level and TSH receptor antibody activity have become normal. This was a prospective randomized study of patients with Graves' disease followed for a mean of 28 months after the withdrawal of antithyroid drug treatment. A total of 174 patients with hyperthyroid Graves' disease were entered consecutively into this study, 11 of whom were subsequently excluded. One hundred and sixty-three patients were divided randomly into two groups having different criteria for the discontinuation of the antithyroid drug. Group 1 consisted of 79 patients whose antithyroid drug treatment was discontinued if both their serum TSH level and thyrotrophin binding inhibitor immunoglobulin (TBII) activity normalized. Group 2 consisted of 84 patients who were treated for 24 months irrespective of their serum TSH level and TBII activity. All patients were treated initially with 400 mg/day of propylthiouracil followed by decreasing doses that maintained a euthyroid state. Serum basal TSH levels and TBII activities were measured using a radioimmunometric assay and a radioreceptor assay, respectively, every 2 months during the antithyroid drug treatment. The remission rate of Group 1 (51.9%) was not significantly different from that (63.1%) of Group 2. The mean duration of treatment in Group 1 was 10.2 +/- 4.5 months (range 5-24, median 10). Median duration of treatment in Group 1 was 14 months shorter than in Group 2. In Group 1, the relapse rate was independent of the duration of the treatment. Further, the mean duration of treatment in the relapsed patients was not significantly different from that for patients who went into remission (9.9 +/- 3.8 vs 10.7 +/- 5.7 months; P greater than 0.1). In Group 2, the relapse rate was dependent on the time when both the serum TSH level and TBII activity were normalized (chi 2 = 27.0, d.f. = 4; P less than 0.001). When both the serum TSH level and TBII activity were normalized, the relapse rate of Group 2 was not significantly different from that of Group I for treatment periods of 6-12 months (25 vs 24.2%, P greater than 0.1), 12-18 months (33.3 vs 40%, P greater than 0.1), and 18-24 months (46.2 vs 50%, P greater than 0.1). However, the relapse rate of Group 2 was significantly lower than that of Group 1 when there was only a 6-month treatment period (5.6 vs 42.9%, P less than 0.05). Of the clinical and laboratory parameters measured, male sex, change of goitre size during treatment, and TSH levels and TBII values at the end of treatment, were of prognostic significance in predicting a relapse. The present study suggests that antithyroid drugs treatment can be terminated more appropriately when both the serum TSH level and TBII activity are made normal, thus avoiding prolonged and unnecessary drug treatment.