Sustained Euthyroidism during Thionamide Antithyroid Drug Treatment: Comparison of Short- and Long-term Methimazole Therapy

 Extended low serum thyrotropin (TSH) levels may increase the risk of cardiovascular events in patients with hyperthyroidism.

 This study aimed to compare the time spent with sustained normal TSH concentration following short- and long-term methimazole treatment.

 A total of 258 patients with Graves’ hyperthyroidism completed 18 - 24 months of methimazole therapy and were randomized to discontinue treatment (n = 128, short-term group) or continue an additional 36 - 102 months of methimazole therapy (n = 130, long-term group). Clinical and laboratory evaluations were performed every 6 months for 132 months after randomization.

 There was no difference in serum-free thyroxine, triiodothyronine, and TSH concentrations between the 2 groups at the time of randomization. Of 128 patients in the short-term group, 5 left in follow-up, 2 became hypothyroid, 67 (54%) had a relapse of hyperthyroidism, and only 54 (44%) were euthyroid at the end of the study. Among 130 patients on the long-term methimazole therapy, 4 were left in follow-up, 24 developed hyperthyroidism, 4 developed hypothyroidism, and 98 (78%) were euthyroid 132 months post-randomization. Total time spent on euthyroidism was 90.4% ± 8.1% of the study period in the short-term and 95.8% ± 7.0% in the long-term treatment groups (P < 0.001). The lowest time spent in euthyroidism (74.6% ± 6.4% of the study period) belonged to 29 (24%) patients in the short-term group under levothyroxine therapy because of fluctuation in serum TSH. Patients in both groups with hyperthyroidism relapse who chose methimazole therapy spent >90% of the study time in euthyroidism.

 In patients with Graves' hyperthyroidism, sustained normal serum TSH levels were more common in the long term as compared to the short-term methimazole treatment.