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 (( 131) IU (24h) ) from( 99m )Tc- pertechnetate thyroid uptake ((TcTU)-Tc-99m ) remains unclear for patients with Graves' disease (GD). This study aims to investigate the feasibility and reliability of (TcTU)-Tc-99m -based( 131)IU(24h) 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 (TcTU20min)-Tc-99m vs (131) IU (24h) 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: (131) IU (24h) 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 (TcTU20min)-Tc-99m in the standard group is described by the following algorithm: estimated (131) IU (24h) = 11.3ln ( (TcTU)-Tc-99m (20min) ) + 39.4 (R2 = 0.62). Based on it, the correlation between estimated and measured (131) IU (24h) 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 (131) IU (24h) was highest in the MWP3 group (95% confidence interval, -15.86 to 15.52%) compared to the MWP1and MWP2 groups. Conclusion: Estimated (131) IU (24h) based on (TcTU)-Tc-99m is not suitable for GD patients with MWP less than 2 weeks at our institution, necessitating further prospective multicenter studies for validation.