Introduction:
Aim: To determine the relationship between changes in serum and fecal zonulin levels depending on the severity of colonic dysbiosis in patients with autoimmune thyroiditis.

Material and Methods:
A total of 143 patients with autoimmune thyroiditis (AIT) were included in the study. An assessment of the species and quantitative composition of the colonic microbiota (CM) was performed in the examined patients. The zonulin level was determined in blood serum and feces using enzyme- linked immunosorbent assay. The α1-antitrypsin (α1-AT) level was also determined in blood serum and feces using ELISA. Based on the results obtained, the α1-AT clearance was calculated. The level of fecal calprotectin (FC) was also assessed using the ELISA method.

Results:
The data obtained indicate that the vast majority of patients had reduced thyroid hormone levels (hypothyroidism) – in 70 (49.0% of those examined). Patients with thyroid hypofunction in AIT were assigned to group 1 of the examined patients. Euthyroidism was diagnosed in 47 (32.9%) patients, who were classified as group 2. Group 3 consisted of 26 (18.1%) patients with AIT who were diagnosed with elevated thyroid hormone levels in their blood serum. As the data show, patients with AIT and thyroid hypofunction are significantly more likely to have grade 3 colonic dysbiosis (p<0.001). It is in patients with AIT and hypothyroidism that grade 4 colonic dysbiosis is significantly more common (in 11.4% of cases – p<0.01), which is 5.0% and 7.6% more than in patients in groups 2 and 3.

Conclusions:
Patients with AIT have been found to have colonic dysbiosis, the severity of which depends on the functional state of the thyroid gland. At the same time, patients with hypothyroidism in AIT are mainly diagnosed with grade 3 CD (in 57.1% – p<0.001), while patients with AIT and euthyroidism and hyperthyroidism are mainly diagnosed with grade 2 CD (in 55.3% (p<0.001) and 46.2% (p<0.01) of patients, respectively). CD (in 55.3% (p<0.001) and 46.2% (p<0.01) of the examined patients, respectively). In patients with AIT, impaired intestinal barrier function was established based on the levels of biomarkers such as zonulin, α1-AT in feces and blood serum, regardless of thyroid function. However, the maximum values of zonulin and α1-AT in feces were diagnosed in patients with AIT with pancreatic hypofunction (an increase in their levels to 168.73±0.51 ng/ml (p<0.001) and to 41.07±0.37 mg/dl(p<0.01), respectively), which directly correlates with the severity of CD in these patients.