Assessment of 20-Minute Tc-99m Thyroid Uptake as a Predictor of 24-Hour I-131 Uptake in a Bangladeshi Patient Cohort

Abstract

Twenty-four-hour radioactive iodine uptake (RAIU) using I-131 is the conventional quantitative standard for thyroid function assessment. However, it requires delayed measurement and exposes patients to higher radiation. Tc-99m pertechnetate thyroid scintigraphy is routinely performed for imaging, but its quantitative uptake potential remains underutilized in Bangladesh. To evaluate the relationship between 20-minute Tc-99m uptake and 24-hour I-131 uptake in an overall Bangladeshi patient cohort and to determine whether Tc-99m uptake can reliably predict I-131 uptake. This cross-sectional study was conducted at National Institute of Nuclear Medicine and Allied Sciences (NINMAS). A total of 120 patients (42 males, 78 females; age 11–73 years) underwent both I-131 uptake measurement and Tc-99m thyroid scintigraphy. Tc-99m uptake was calculated using standardized region-of-interest (ROI) analysis from gamma-camera images. Descriptive statistics, paired t-test, Pearson correlation, regression analysis, and diagnostic concordance evaluation were performed. The mean 20-minute thyroid uptake of the administered Tc-99m dose was 2.19%, while the mean 24-hour thyroid uptake of the administered I-131 dose was 17.63%. Tc-99m uptake significantly underestimated I-131 uptake (p < 0.001). A moderate positive correlation was observed (r = 0.77, p < 0.001; R² = 0.60). Importantly, 20-minute Tc-99m uptake was a reliable predictor of 24-hour I-131 uptake. Diagnostic concordance between the methods was 88.33%, and a stronger correlation was seen in females, with sex-specific models further enhancing predictive accuracy. Twenty-minute Tc-99m uptake demonstrates correlation and high diagnostic agreement with 24-hour I-131 uptake. It may serve as a rapid, low-radiation, and cost-effective alternative for preliminary thyroid function assessment in Bangladesh.

Ban. J. Med. Phys., Vol -17, Issue -1, 2026 : 112-118