# Assessment of Iodine Status in Pregnant Women: Diagnostic Performance of Spot Urinary Iodine Indices Compared with 24-h Urinary Iodine Excretion

## TL;DR

This study finds that many pregnant women in Turkey have low iodine levels, and creatinine-adjusted urine tests may better assess iodine status than single urine samples.

## Contribution

The study compares the diagnostic performance of creatinine-adjusted spot urine tests with 24-hour urine collections for assessing iodine status in pregnant women.

## Key findings

- 70% of pregnant women had 24h-UIE below the Estimated Average Requirement.
- Creatinine-adjusted UIC showed stronger correlation with 24h-UIE than unadjusted UIC.
- Subclinical hypothyroidism was present in 16.3% of participants, but no overt hypothyroidism was observed.

## Abstract

Background: Adequate iodine intake during pregnancy is essential for optimal maternal thyroid function and fetal neurodevelopment. Although universal salt iodization has been implemented in Turkey, pregnant women may remain vulnerable to iodine insufficiency. This study aimed to evaluate maternal thyroid function in relation to iodine status, and to comprehensively compare the diagnostic performance of spot urinary indices and creatinine-adjusted measures against measured 24 h urinary iodine excretion (24h-UIE) in pregnant women. Methods: A total of 227 pregnant women attending family health centers in Afyonkarahisar, Turkey, provided both spot urine samples and complete 24 h urine collections. Urinary iodine concentration (UIC), creatinine-corrected UIC (UIC/UCr), and 24h-UIE were measured. Thyroid function tests were interpreted using trimester-specific reference ranges. Correlations between urinary indices were assessed, and ROC analyses were performed using 24h-UIE as the operational reference. A structured questionnaire evaluated iodine-related dietary knowledge and salt-use practices. Results: The median spot UIC was 59.0 µg/L, indicating insufficient recent iodine intake at the population level. Based on 24h-UIE, 70% of participants had excretion levels below the Estimated Average Requirement (EAR) threshold (<144 µg/day). Spot UIC showed a weak correlation with 24h-UIE (rho = 0.270, p < 0.001), whereas UIC/UCr demonstrated a stronger correlation (rho = 0.491, p < 0.001). In ROC analyses, UIC/UCr yielded a significantly higher AUC than spot UIC (0.774 [95% CI: 0.707–0.841] vs. 0.670 [95% CI: 0.593–0.748]; DeLong p = 0.016). Overt hypothyroidism was not observed; subclinical hypothyroidism was present in 16.3% of participants. While no overall association was found between iodine indices and thyroid status, in the first trimester, those with subclinical hypothyroidism had higher 24h-UIE medians than euthyroid peers (134.2 vs. 100.3 µg/day, p = 0.037), although both groups remained below the EAR threshold. Knowledge regarding iodine-rich foods and iodized salt use was limited among the study population. Conclusions: Iodine insufficiency remains highly prevalent among pregnant women in this region despite salt iodization. While spot UIC alone showed limited agreement with 24h-UIE, creatinine-adjusted UIC may offer improved interpretability under conditions of variable urine dilution. Preserved thyroid function in the presence of iodine insufficiency highlights the silent nature of this condition during pregnancy. Strengthened pregnancy-specific iodine surveillance and targeted antenatal education are warranted.