Dietary iodine and selenium exposure and thyroid function in adults: a total diet study with biomarker assessment

Excessive intake of iodine and selenium is increasingly recognized as a potential public health concern in populations undergoing dietary transition, yet integrated evidence linking food-based exposure, biomarkers and thyroid function remains limited. This study evaluated dietary iodine and selenium exposure and examined their association with thyroid function in healthy adults in Halabja City, Kurdistan Region, Iraq, using a combined dietary and biomarker approach.

A cross-sectional study was conducted among 201 healthy adults (18-60 yrs) from Halabja City, Kurdistan Region, Iraq. Dietary intake was assessed using a semi-quantitative food frequency questionnaire combined with a total diet study approach, in which composite food samples were analysed by inductively coupled plasma-mass spectrometry. Urinary iodine (iodine-to-creatinine ratio), serum selenium and thyroid-stimulating hormone (TSH) were measured. Multivariable regression models were used to examine associations between iodine intake and TSH after adjustment for demographic and nutritional covariates.

Mean iodine and selenium intakes were 206.99 µg/day and 291.14 µg/day, respectively. Cereal and cereal-based foods were the major contributors to both iodine and selenium intake, accounting for 65.97% and 68.82% of total intake, respectively. Biomarker findings indicated adequate-to-high iodine and selenium status at the population level. Most participants remained within the euthyroid range, and dietary iodine intake was not independently associated with TSH after multivariable adjustments.

This study suggests novel evidence that staple-based dietary patterns can drive excessive iodine and selenium exposure in adults. Although no independent association between dietary iodine intake and TSH was observed after multivariable adjustment, most participants remained within the euthyroid range, suggesting possible physiological adaptation; however, subclinical effects cannot be excluded. These findings highlight the importance of monitoring iodized salt practices and dietary exposure pathways in populations with reliance on staple foods, and demonstrate the value of integrating total diet study methodology with biomarker assessment into nutritional surveillance.