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

Abstract

Purpose: 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.

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: 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.

Results: 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.

Conclusion: 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.