Background. Vitamin D, known for its role in calcium-phosphorus homeostasis, is also a significant immunomodulatory factor. Vitamin D deficiency has been reported in some autoimmune disorders. Recently, vitamin D level in autoimmune thyroiditis (HT -Hashimoto's thyroiditis) has become the subject of researchers' interest. Objectives. This study aims to assess vitamin 25-OH-D3 levels in HT patients in comparison to a control group in the Polish population. This would be the first attempt

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... d in this region with such poor sunlight exposure. Material and Methods. The group we studied consisted of 62 subjects diagnosed with HT (mean age 49.15 ± 15.51) and 32 healthy controls matched with age and sex (mean age 46.09 ± 14.32). All blood samples were collected in the first quarter of the year to minimize the impact of seasonal fluctuations of vitamin D concentrations. Results. In the HT group the mean vitamin D level was 20.09 nmol/L (SD ± 12.66), compared to 30.31 nmol/L (SD ± 19.49) in the controls, p = 0.014. None of the patients and the controls was vitamin D sufficient (75-125 nmol/L). The deficiency (< 50 nmol/L) was significantly more common among HT patients compared to the controls (61-98.4% vs. 27-84.4%, p = 0.029). Conclusions. In conclusion, we found that serum vitamin D concentration is significantly lower in HT patients in comparison to the control group. This suggests vitamin D deficit as one of the risk factors for HT development. Observed vitamin D level was also low in the control group, therefore wider supplementation in general population should be recommended (Adv Clin Exp Med 2015, 24, 5, 801-806).