Association between vitamin D levels and thyroid function tests in euthyroid women with obesity

ObjectiveThyroid hormones are essential for energy expenditure and metabolism, and their dysfunction is associated with obesity. Vitamin D, besides its role in bone health, may influence thyroid function, especially in women with obesity, where deficiency is common and linked to altered thyroid hormone levels and sensitivity. This study aimed to investigate the relationship between vitamin D levels and thyroid function tests (TFTs) in euthyroid women with obesity.MethodsThis single-center, cross-sectional study was conducted in the obesity center of a tertiary hospital in Turkey, between July and September 2024. Women with a body mass index of 30 or higher and confirmed to be euthyroid based on TFTs were included, while those receiving thyroid medications or lacking concurrent 25-hydroxyvitamin D (25(OH)D) measurements were excluded. Data were collected retrospectively from the hospital's information management system, and participants were categorized into three groups based on their 25(OH)D levels (<20 ng/mL, 20-29.9 ng/mL, ≥ 30 ng/mL).ResultsThe study included 158 participants. No significant associations were observed between thyroid stimulating hormone (TSH) levels and participant characteristics or 25(OH)D levels. In contrast, thyroxine (T4) levels were significantly higher in participants with diabetes mellitus (DM), those without psychiatric disorders, and those with 25(OH)D levels of 30 ng/mL or higher compared to others. ANCOVA analysis adjusted for DM and psychiatric disorders confirmed significant differences in T4 levels among the 25(OH)D groups.ConclusionThis study suggests that vitamin D levels may be associated with T4 levels in euthyroid women with obesity. Although no link was found with TSH, the findings suggest that further research is needed on the potential role of vitamin D in thyroid function. These preliminary findings may also highlight the clinical relevance of monitoring and managing vitamin D levels in individuals with obesity.