Objective: To assess iodine status in adults with euthyroid in Aydın province using spot urinary iodine concentration (UIC) and the urinary iodine-to-creatinine (I/Cr) ratio and to evaluate the relationship between serum thyroglobulin (Tg) and iodine status.

Methods: This retrospective study included 162 euthyroid and anti-Tg-antibody-negative adults evaluated between 2023 and 2025, with simultaneous spot UIC, urinary creatinine, and serum Tg measurements. Participants were classified per World Health Organization (WHO) criteria as iodine-deficient (UIC <100 µg/L) or iodine-sufficient (UIC ≥100 µg/L). Serum Tg was compared between groups using the Mann–Whitney U-test, and its relationship with UIC and the I/Cr ratio was assessed by Spearman correlation.

Results: Of the 162 participants, 123 (75.9%) were iodine-deficient and 39 (24.1%) were iodine-sufficient; median UIC and serum Tg for the whole group were 63.0 µg/L and 13.27 ng/mL, respectively. Median serum Tg was significantly higher in the iodine-deficient group than in the iodine-sufficient group (15.11 vs 9.49 ng/mL; U= 1807.5, P=.021). Serum Tg showed weak but significant negative correlations with both spot UIC (rho=−0.184, P=.019) and the I/Cr ratio (rho=−0.205, P=.009).

Conclusion: Despite weak individual-level correlations with urinary iodine measures, serum Tg was significantly elevated in iodine-deficient participants, supporting its potential role as a population-level biomarker of iodine status rather than an individual diagnostic tool. Given the lack of validated Tg reference values for adults, these findings underscore the need for locally derived thresholds and further validation in larger, prospective cohorts.