[Relationship of vitamin A and thyroid function in Arctic residents]

At the present time the prevalence of thyroid diseases keeps growing, so knowledge of the factors affecting thyroid activity is very important. Vitamin A (retinol) is a fat-soluble vitamin with a hormone-like effect that can influence both the expression of thyroidstimulating hormone in the pituitary gland and the synthesis of thyroid hormones. The aim of the research was to study vitamin A serum level and its relationship with the thyroid profile in residents of the Arctic zone of the Russian Federation, depending on gender. Material and methods. In the course of a single-center observational cross-sectional study, 304 apparently healthy residents of villages in the Arkhangelsk region (103 men and 201 women) were examined. The serum concentration of vitamin A was determined by the fluorometric method, and the content of thyroid parameters was determined by the enzyme immunoassay. The integral thyroid index (ITI) and the index of peripheral conversion of iodothyronines (IPC) were calculated. The subjects were divided into 4 groups depending on the quartile of vitamin A. The relative risk of developing subclinical hypothyroidism was calculated with a reduced concentration of vitamin A. Results. The content of vitamin A was 1.97 (1.18; 2.97) μmol/l. Retinol levels in the female population were significantly lower than in the male population (1.85 vs 2.27 μmol/l, p<0.0001). Women in group 1 compared with group 4 showed significantly higher levels of thyrotropin (2.33 vs 1.40 μIU/ml, p=0.0002) and thyroxin (109.8 vs 99.4 nmol/l, p=0.010), while the ITI values, on the contrary, were the lowest (8.12 and 13.85 c.u., p=0.0002). The relative risk calculation showed that vitamin A levels below 1.39 μmol/L increase the risk of subclinical hypothyroidism in women by 2.01 times [95% confidence interval 1.07-3.78]. Men in group 4 compared with group 1 showed a lower content of thyroxin (83.0 vs 109.2 nmol/l, p=0.009) against the background of higher IPC values (0.019 vs 0.016 c.u., p=0.046). Conclusion. In the inhabitants of the Arctic, the content of vitamin A is within the reference values, however, in 24.3% of men, its level is above the norm. In women, the concentration of retinol is significantly lower, and its level of less than 1.39 μmol/l increases the risk of developing subclinical hypothyroidism by 2.0 times. Increased values of vitamin A in the inhabitants of the North cause lower levels of thyroxine and an increase in the peripheral conversion of iodothyronines, which is more pronounced in men.