Introduction: Autoimmune thyroid diseases (AITD), including Hashimoto’s thyroiditis (HT), are the most common organ specific autoimmune disorders. Vitamin D (vit-D) is a steroid molecule, mainly produced in the skin, which regulates the expression of many genes.

The vitamin D receptor (VDR) is found in most tissues and cells in the body. Many studies suggests that vit-D deficiency, which is common worldwide, could also play an important role in autoimmune diseases, including HT. The aim of our study was to show the potential differences in vit-D levels between healthy women and individuals with hypothyroidism and HT. Additionally, we assessed the correlation between vit-D concentration and the level of TSH and anti-thyroid antibodies in females diagnosed with HT.

Material and methods: The study group included 370 subjects. The group was divided into 3 subgroups: (125 — healthy individuals, 111 — hypothyreosis, 134 — HT). Anthropometric measurements including height and weight were obtained in all participants. Body mass index (BMI) was calculated as body weight (in kilograms) divided by the square of body height (in metres). The measurement of the thyroid gland was performed using an ultrasound scan with a 10-MHz linear probe by one endocrinologist (Vivid S60N).

Results: We noticed that a lower level of vit-D was connected with a higher level of TSH in each subgroup. There was also strong, negative correlation between TSH and vit-D levels in all the study groups. Moreover, there was a weak, negative correlation between antithyroid peroxidase antibody (anti-TPO) and antithyroglobulin antibody (anti-TG) and vit-D levels in females with HT regardless of vit-D status: < 20 ng/mL, 20–30 ng/mL, and > 30 ng/mL.

Conclusions: To our knowledge, the current study is the first in Poland to compare vit-D status in healthy patients and patients with hypothyroidism, taking into account the level of antibodies (anti-TPO and anti-TG). The results of our study suggest that vit-D supplementation in patients with hypothyroidism, especially in the course of AITD, although determining its optimal, safe dose requires further research.