Gender differences in estimating I-131 thyroid uptake from Tc-99m thyroid uptake for benign thyroid disease

Objective: For radioactive Iodine-131 (131I) treatments of thyroid diseases, increased efficacy has been reported for personalized dosimetry treatments. The measurement of Iodine-131 thyroid uptake (131IU) is required in these cases. This study aims to investigate whether 99mTc thyroid uptake (99mTcU) may be used in place of 131IU for implementing personalised treatments. Methods: A retrospective study of 152 benign thyroid disease 131I treatments was carried out during 2012–2020; 117 treatments were for female patients while 35 were for male patients diagnosed with either Graves’ disease, multinodular goitre or toxic nodules. Results: A statistically significant correlation was found between 131IU and 99mTcU data, with the data more correlated for male than female patients (r = 0.71 vs 0.38, p-value < 0.001). Patient age and time difference between the two respective uptake measurements significantly influenced the uptake correlation in females but not for the male cohort, although there was no significant difference between the parameters across gender. Thyroid diagnosis and hormone levels showed a significant correlation with uptakes in both genders. Estimating 131IU based on 99mTcU was shown to be predictive for male but not in female patients (R2 = 91% vs 16%). Conclusion: Estimating 131IU based on 99mTcU is not recommended for females at our centre. Males reported good correlation, but a larger sample would be needed for validation. Advances in knowledge: The initial findings showed a significant gender difference in benign thyroid uptake parameters at our centre, highlighting the potential need for gender consideration when planning 131IU patient management and when reporting studies results.