Biofluid iodine concentrations are associated with mild cognitive impairment: The potential mediating role of thyroid hormones.

BACKGROUND: Iodine nutritional status is associated with mild cognitive impairment (MCI) in older adults. However, evidence comparing iodine concentrations across multiple biofluids and evaluating the potential mediating role of thyroid function remains limited. METHODS: This cross-sectional study was conducted in three regions of China with varying levels of iodine nutrition. Drinking water, urine, saliva and serum samples were collected from people aged ≥60, and the iodine concentrations in the water, urine, saliva and serum were determined using ICP-MS, and 24-h urinary iodine excretion (24 h-UIE) was calculated. Cognitive function was assessed using a combination of the Mini-Mental State Examination (MMSE), the Activities of Daily Living (ADL) scale, and a structured questionnaire for cognitive-related medical history. RESULTS: Among 3888 participants, 1319 (33.9%) had MCI. Iodine status and cognitive outcomes differed markedly across regions. Higher levels of several biofluid iodine indicators were associated with lower MMSE scores. Among urinary measures, 24-h urinary iodine concentration (24 h-UIC) showed the strongest inverse association (β = -1.764, 95% CI: -1.973 to -1.555). Region-stratified analyses revealed substantial heterogeneity in iodine-cognition associations (all P for interaction <0.05). TSH was inversely associated with MMSE and partially mediated the associations of 24 h-UIE and 24 h-UIC with MCI, accounting for 10.23% (ACME OR = 1.012, 95% CI: 1.002-1.022) and 8.90% (ACME OR = 1.012, 95% CI: 1.001-1.024) of the total associations, respectively. These findings remained robust in sensitivity analyses. CONCLUSION: Our results indicate that in older adults, biofluid iodine concentrations are associated with MCI, and this association is only partially mediated by TSH.