The study examined vitamin D levels in 75 patients with autoimmune thyroiditis (AIT) with diminished thyroid function. The objective of the study — to determine the content of 25(OH)D in patients with overt and subclinical hypothyroidism on the background of AIT. Materials and Methods. 75 patients (59 women and 16 men) and 25 apparently healthy subjects (matched for age and sex) were followed. The levels of thyroid stimulating hormone (TSH), free thyroxine (fT4), free triiodothyronine (fT3),

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... ibodies to thyroid peroxidase (TPO), and the content of 25(OH)D in the blood serum were determined. Results. Patients with AІT were divided according to the levels of TSH and thyroid hormones into the group with subclinical (n = 21) and overt (n = 54) hypothyroidism. TSH level was significantly higher in the group with subclinical (6.80 ± 1.84 mcU/ml) and overt hypothyroidism (11.38 ± 2.16 mcU/ml) compared with the control group (2.11 ± 0.37 mcU/ml) (p < 0.05). The level of antibodies to TPO was 312.83 ± 7.19 U/ml in subclinical hypothyroidism and 546.29 ± 9.81 U/ml — in overt hypothyroidism. 25(OH)D level in blood serum was significantly lower in subclinical (18.8 ± 1.2 nmol/l) and overt hypothyroidism (21.7 ± 1.3 nmol/l) compared with the control group (27.3 ± 1.4 nmol/l) (p < 0.05). Highly reliable negative correlation was established between the level of TSH, antibodies to TPO and vitamin D (p < 0.001). Also highly reliable positive correlation was established between the levels of serum 25(OH)D and fT4 content (p < 0.001), between TSH and TPO antibody levels (p < 0.05). Conclusions. Vitamin D deficiency is combined with the presence of AIT with diminished thyroid function (in subclinical and overt hypothyroidism). Further studies are needed to determine vitamin D deficiency as a causal factor of AIT.