Time to Normalization and Sustainable Normal Serum Thyrotropin Concentrations in Patients with Hyperthyroidism: Comparison of Methimazole and Radioactive Iodine Treatments.

ABSTRACT

OBJECTIVE: The aim of this study was to compare the "time to euthyroidism" and "time spent in euthyroidism" following methimazole (MMI) and radioactive iodine (RAI) treatments. METHODS: Three hundred fifty-eight patients with hyperthyroidism, 178 who underwent long-term MMI treatment and 180 patients who underwent RAI treatment, were analyzed. The time to normalization of increased serum values of free thyroxine and triiodothyronine and suppressed serum thyroid-stimulating hormone (TSH) values as well as the percentage of time that the thyroid hormone levels remained within normal ranges during a mean follow-up time of 12 years were compared. RESULTS: The mean time to euthyroidism was 4.59 ± 2.63 months (range, 2-16 months) in the MMI group and 15.39 ± 12.11 months (range, 2-61 months) in the RAI group (P < .001). During follow-up, the percentage of time spent in euthyroidism was 94.5% ± 7.3% and 82.5% + 11.0% in the MMI and RAI groups, respectively (P < .001). Serum TSH values above and below the normal range were observed in 5.3% and 0.2% of patients, respectively, in the MMI group and 9.8% and 7.7% of patients, respectively, in the RAI group (P < .001). The time to euthyroidism and the percentage of time spent in euthyroidism in 40 RAI-treated patients with euthyroidism were similar to those in the MMI group and significantly shorter than those in the RAI-treated hypothyroid and relapsed subgroups. In patients who continued MMI therapy for >10 years, the percentage of time spent in euthyroidism was >99%. CONCLUSION: In our cohort of selected patients, MMI therapy was accompanied by faster achievement of the euthyroid state and more sustained normal serum TSH levels during long-term follow-up compared with RAI therapy.