SERUM THYROGLOBULIN LEVELS ARE PREDICTIVE OF URINARY IODINE CONCENTRATION THRESHOLDS FOR DEFINING POPULATION IODINE STATUS.

Thyroglobulin (Tg) has been considered a measure of iodine status but there is no global guidance. This analysis examines relationship between serum Tg and spot urinary iodine concentration (UIC) data from Guatemala's 2018-2019 Epidemiological Health and Nutritional Surveillance System and identify Tg concentrations that correspond to current WHO thresholds for population iodine status among women ages 15-49 years. We analyzed data from non-pregnant Guatemalan women aged 15-49 years who had both UIC and Tg measurements. Correlations were examined. Bootstrap stratified finite sampling with replacement was used to generate cluster k-medians of UIC (mUIC) and Tg (mTg) that served as population unit of analyses. Non-linear restricted cubic spline regression dose-response curve functions and ordinary differential equations were then used to derive Tg threshold corresponding to WHO definitions for UIC. Serum Tg and UIC data were available for 730 non-pregnant women. Mean age was 30.2 ± 9.3 years. mTg was 10.4 ng/mL (9.9, 10.8) and mUIC was 148.7 μg/L (139.1, 161.0). Correlations between spot UIC and Tg were not significant at individual-level, but correlations based on population k-medians were significant (Spearman r = -0.21 to -0.06 each p<0.0001) and demonstrated U-shaped relationship according to WHO categories. Derived mTg cut-offs were 14.2 ng/mL predictive of UIC insufficiency, 10.2 ng/mL for UIC adequacy, 8.5 ng/mL for UIC above adequate and 10.8 ng/mL for UIC excess. The significant and graded mUIC-mTg correlations suggests that Tg concentrations predictive of UIC categories are obtainable for non-pregnant Guatemalan women aged 15-49 years. The newly derived mTg cut-off may be more discriminant at lower spectrum of UIC in terms of identifying iodine deficient women more so than those in the UIC excess category. Further studies could inform if Tg could be a potentially viable biomarker for assessing population iodine status in women.