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 the relationship between serum Tg and spot urinary iodine concentration (UIC) data to identify Tg concentrations that correspond to current WHO thresholds for population iodine status. We analysed data from 730 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 <i>k-medians</i> of UIC (mUIC) and Tg (mTg) that served as the population unit of analyses. Non-linear restricted cubic spline regression dose-response curve functions and ordinary differential equations were then used to derive the Tg threshold corresponding to WHO definitions for UIC. Mean age was 30·2 (sd 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 NS at the individual level, but correlations based on population <i>k-medians</i> were significant (Spearman <i>r</i> = -0·21 to -0·06, each <i>P</i> < 0·0001) and demonstrated a <i>U</i>-shaped relationship according to WHO categories. Derived mTg cutoffs 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 suggest that Tg concentrations predictive of UIC categories are obtainable for non-pregnant Guatemalan women aged 15-49 years. The newly derived mTg cutoff may be more discriminant at a lower spectrum of UIC in terms of identifying iodine-deficient women, more so than in the UIC excess category.