Effect of Methimazole Withdrawl Period on the I-131 Uptake Estimation Using Tc-99 m Thyroid Scanning in Graves’ Disease

PURPOSE: The effect of methimazole withdrawal period (MWP) on the estimation of 24-hour-radioiodine thyroid uptake (131IU24h) from 99mTc-pertechnetate thyroid uptake (99mTcTU) remains unclear for patients with Graves' disease (GD). This study aims to investigate the feasibility and reliability of 99mTcTU-based 131IU24h estimation with different MWPs. METHODS: We enrolled 116 GD patients scheduled for 131I therapy at our hospital between April 2022 and April 2023. Based on MWP, the patients were categorized as standard (no methimazole or MWP > 1 month), MWP1 (MWP ≤ 1 week), MWP2 (MWP > 1 week to ≤2 weeks), and MWP3 (MWP > 2 weeks to <1 month). Fisher's exact test, one-way ANOVA, or Kruskal-Wallis test were used to compare variables. Fitted curves of 99mTcTU20min vs 131IU24h were plotted for the standard group. Linear relationships and Bland-Altman plots were used to illustrate the relationship and consistency between estimated and measured 131IU24h. RESULTS: 131IU24h was higher in the MWP1 group compared to MWP2 (70.22 ± 7.95% vs 61.92 ± 9.84%, P = .001), and thyroid mass was greater in the MWP1 group (36.15 ± 22.38 g) vs MWP3 (21.25 ± 11.90 g, P = .005). The relationship between 131IU24h and 99mTcTU20min in the standard group is described by the following algorithm: estimated 131IU24h = 11.3ln (99mTcTU20min) + 39.4 (R2 = 0.62). Based on it, the correlation between estimated and measured 131IU24h was weak in MWP1 and MWP2 (both P > .05) but strong in MWP3 (r = 0.66, P = .002). Additionally, the agreement between estimated and measured 131IU24h was highest in the MWP3 group (95% confidence interval, -15.86 to 15.52%) compared to the MWP1and MWP2 groups. CONCLUSION: Estimated 131IU24h based on 99mTcTU is not suitable for GD patients with MWP less than 2 weeks at our institution, necessitating further prospective multicenter studies for validation.