Effect of methimazole withdrawl period on the I-131 uptake estimation using Tc-99m thyroid scanning in Graves' disease.

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The effect of methimazole withdrawl 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. 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: standard (no methimazole or MWP>1 month), MWP1 (MWP≤1 week), MWP2 (MWP>1 week to ≤2 weeks) and MWP3 groups (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 versus 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. 131IU24h was higher in MWP1 group compared to MWP2 (70.22±7.95% vs 61.92±9.84%, P = 0.001), and thyroid masswas greater in MWP1 group (36.15±22.38g) versus MWP3 (21.25±11.90g, P = 0.005). The relationship between131IU24h and 99mTcTU20min in the standard group is described by the 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 groups (both P>0.05) but strong in MWP3 (r=0.66, P=0.002). Additionally, the agreement between estimated and measured 131IU24h was highest in the MWP3 group (95% CI, -15.86 to 15.52%), compared to the MWP1and MWP2 groups. Estimated 131IU24h based on 99mTcTU is not suitable for GD patients with MWP less than 2 weeks at our institution, necessitating further prospective multi-center studies for validation.