Low 25 Hydroxyvitamin D Levels are Independently Associated with Autoimmune Thyroiditis in a Cohort of Apparently Healthy Overweight and Obese Subjects

Low vitamin D levels have been associated with autoimmune disorders and, then, with the Hashimoto's autoimmune thyroiditis (AT), the most common autoimmune disease. Obesity is characterized by lower vitamin D levels and higher risk to develop autoimmune diseases. The aim of the study was to examine the possibility of an association between AT and decreased 25(OH) vitamin D (25(OH)D) levels in a cohort of otherwise healthy overweight and obese subjects.Two hundred sixty one overweight subjects (mean age: 40.9 + 15.6 years, 200 women and 61 men) were enrolled for this study. All of them did not show any clinically evident metabolic or chronic diseases (i.e. hypertension, diabetes mellitus, renal failure, etc.) and did not use any kind of drug. Serum fasting levels of 25(OH)D, anti-thyroid peroxidase (TPO-Ab) and antithyroglobulin (TG-Ab) antibodies, free triiodothyronine (FT3), free thyroxine (FT4), thyroid stimulating hormone (TSH), glucose, uric acid and lipids (triglycerides, total, HDL and LDL cholesterol) were measured. Demographic, anthropometric and clinical parameters (age, body mass index (BMI), waist circumference, blood pressure) were also assessed.Fifty five percent of all subjects (144/261) showed vitamin D deficiency (≤ 20 ng/ml), and 17% of all individuals had AT (45/261). The percentage of subjects having vitamin D deficiency was significantly higher among those with AT (31 of 45, 69%) than in those without AT (113 of 216, 52%) (χ2= 4.1, p = 0.042). TSH levels were significantly higher in subjects with AT as compared to those without AT (M-W = 7715.5, p < 0.0001). The final logistic model of a multivariate analysis, performed with AT as the dependent variable and sex, age, BMI category, 25(OH)D category, and HDLcholesterol and TSH levels as the independent ones, showed that patients with AT were more likely to have deficiency of 25(OH)D (p = 0.031) and higher TSH (p < 0.005) levels. Seventy six percent of patients with vitamin D deficiency (110 of 144) were obese whereas 59% of patients without vitamin D deficiency were obese (69 of 117) (p=0.003). Waist circumference was different between subjects with deficiency or normal 25 (OH) D levels (p=0.016).This study clearly shows that vitamin D deficiency is significantly associated to AT in overweight and obese subjects and confirms that obesity is associated to lower vitamin D circulating levels. We suggest that screening for AT should be suggested in all obese subjects with vitamin D deficiency and that vitamin D deficiency should be researched in all obese subjects with AT.