Vitamin D is a steroid prohormone, mainly synthesized in the skin, which, after conversion to an active metabolite, regulates calcium and phosphorus metabolism and bone homeostasis. Currently, multiple data suggest, that it has also many extra-skeletal actions. Vitamin D deficiency may play an important role in pathogenesis of infections, autoimmune diseases, metabolic syndromes, cardiovascular diseases, cancers and all-cause mortality (1, 2). In recent years there have been many studies published, which showed vitamin D associations with autoimmune thyroid diseases, including Hashimoto’s thyroiditis (HT), postpartum thyroiditis (PPT) and Graves’ disease (GD) (3, 4).

Vitamin D occurs in two different forms, as cholecalciferol (vitamin D₃) and ergocalciferol (vitamin D₂). Cholecalciferol is mainly synthesized in the skin upon exposure to ultraviolet B radiation by 7-dehydrocholesterol reductase (DHCR7), but it can be also obtained from few dietary sources (mainly fatty fish). Ergocalciferol is derived from dietary sources – it is synthesized by plants and fungi (2). Both forms are transported to the liver and there hydroxylated to 25-hydroxyvitamin D (25(OH)D₃, calcidiol) by 25-hydroxylase (CYP27A1, CYP2R1), which has little biological activity, but is the main storage form of vitamin D. Calcidiol is converted to active hormone, calcitriol (1,25(OH)₂D₃) by 1α-hydroxylase (CYP27B1), which is mainly expressed in kidney. This enzyme is stimulated by parathormone (PTH) and inhibited by high 1,25(OH)₂D₃ concentration and fibroblast growth factor 23 (FGF23). Calcitriol is inactivated by 24-hydroxylase (CYP24A1) (1, 2). Calcitriol is also produced from calcidiol by several other cell types (including immune cells), which express 1α-hydroxylase, without the above regulatory feedback, as an autocrine or paracrine cytokine (1). The main factor determining extrarenal 1,25(OH)₂D₃ synthesis seems to be 25(OH)D₃ concentration (5). There have been found many associations between serum calcidiol level, rather than calcitriol concentration, and extraskeletal health outcomes (5). Both calcidiol and calcitriol are highly hydrophobic molecules, stored in the adipose tissue and circulating in blood bound mainly to vitamin D binding protein (DBP) (85 and 88%, respectively) and, with lower affinity, to albumin (15 and 12%, respectively) (6, 7). Less than 1% of calcitriol is free and can bind to nuclear vitamin D receptor (VDR), which acts on vitamin D response element (VDRE) of multiple target genes to exert its effects (8). VDR is found in most cells and tissues, including the thyroid (1, 8, 9). 1,25(OH)₂D₃ decreases cellular proliferation, induces differentiation and apoptosis, influences angiogenesis, and modulates the immune system (1, 2). A membrane-bound VDR has been also hypothesized, which would mediate rapid, non-genomic actions of 1,25(OH)₂D₃ (8).

Vitamin D is a potent immunomodulator. Most immune cells, including macrophages, antigen-presenting cells (APCs), lymphocytes T and B, express not only VDR, but also 1α-hydroxylase (2, 10). Generally, vitamin D activates the innate system and regulates the acquired immune response (2, 11). It inhibits major histocompatibility complex class II molecules expression on dendritic cells surface and modulates cytokine secretion, shifting the balance from a Th1 and Th17 to a Th2 phenotype (2, 10). Calcitriol also inhibits B cell proliferation, differentiation of B cells into plasma cells, immunoglobulin secretion and formation of memory B cells, as well as induces B cell apoptosis (2, 10). To summarize, vitamin D promotes immunotolerance, and, therefore, could be beneficial in autoimmune disorders (1, 2, 10).

Not only vitamin D status, but also polymorphism of genes involved in vitamin D metabolism, transport and activity was shown to be associated to susceptibility to autoimmune disorders (10).

This review presents the current data regarding the role of vitamin D in autoimmune thyroid diseases.

The thyroid gland secretes mainly thyroxine (T4) and, in less extend, triiodothyronine (T3), which are crucial for maintaining specific function of multiple cell types and tissues and stimulating metabolism. Thyroid function is controlled by thyrotropin (thyroid stimulating hormone – TSH) released by the pituitary. The data on associations between vitamin D status and TSH or thyroid hormone concentrations are very limited and often divergent.

Experiments on rats showed, that those fed with severely vitamin D deficient diet had lower TSH, but similar T4 levels to vitamin D sufficient animals. However, administration of high doses of calcitriol did not influence TSH or T4 concentrations (12). In streptozotocin-induced diabetic rats inhibited peripheral conversion of FT4 into FT3 secondary to reduction in deiodinase 2 (D₂) expression was observed. Vitamin D greatly corrected the alterations in thyroid profile and D₂ expression (13).

In human, when hospitalized patients without history of thyroid diseases were studied, TSH levels did not differ between those with 25(OH)D₃ very low (≤ 10 ng/ml) and high (≥ 40 ng/dl) concentrations (9). On the contrary, in two normal population-based studies, high vitamin D status was associated with lower TSH levels. In a Thai cohort it was observed only in young subjects (14), while in Chinese – also in middle-aged and elderly ones (15). In postmenopausal women suppressed TSH was also associated with higher vitamin D levels, however the relationship was not linear (16). On the other hand, in a population of euthyroid adults Barchetta et al. showed a strong association between vitamin D deficiency and higher TSH levels (p = 0.01) (17). Those and other authors suggested, that reported seasonality in TSH secretion was associated with vitamin D status (17, 18).

Hypothyroidism was, according to Mackawy et al., as well as to Kim, associated with hypovitaminosis D (19, 20), however others did not confirm these observations (21, 22).

Bouillon et al. reported, that in hypothyroid patients serum calcidiol levels were comparable to normal subjects, while calcitriol concentrations were significantly increased (73 ± 28 ng/l vs. 42 ± 13 ng/l in control subjects; p < 0.001), probably secondary to high parathyroid hormone levels (53 ± 17 mU/l vs. 26 ± 9 mU/l in controls; p < 0.001) (21).

Very recently, in a large retrospective Canadian cohort study regarding the influence of vitamin D supplementation on thyroid function, baseline mean 25(OH)D₃ concentration was significantly lower in hypothyroid subjects than in healthy controls (27.2 vs. 32.8 ng/ml) (18). Interestingly, in type 2 diabetic patients Calvo-Romero and Ramiro-Lozano found slightly higher serum thyrotropin levels in vitamin D deficient subjects (at the limit of statistical significance), however, with no effect of correction of vitamin D deficiency on TSH concentrations (23).

In untreated hyperthyroid patients unaltered calcidiol, low calcitriol and high 24,25(OH)₂D₃ concentrations were observed (21, 24, 25). It was explained to be a result of high bone turnover causing secondary hypoparathyroidism or of competitive inhibition of calcitriol synthesis by 24,25(OH)₂D₃.

However, recently, in women with gestational transient thyrotoxicosis 25(OH)D₃ concentrations were found to be significantly lower than in pregnant female with normal thyroid function (26, 27). Moreover, Pan et al. showed, that calcidiol levels correlated positively with TSH and negatively with FT4 and FT3 concentrations (27).

Autoimmune thyroid diseases are the most common organ-specific autoimmune disorders. Hashimoto thyroiditis, also known as chronic lymphocytic thyroiditis, is a typical T-cell mediated autoimmune disease, in which intrathyroidal infiltration of B and T lymphocytes is observed. It is well established, that type 1 T helper (Th1) lymphocytes participate in the development of HD. Cytokines secreted by Th1 cells activate cytotoxic T-lymphocytes and natural killer (NK) cells, leading to thyrocyte destruction. Nowadays increasing data indicate important role of other mechanisms in pathogenesis of HT (including autoantibodies, other subgroups of T helper cells such as Th17, regulatory T cells, disturbances of the process of apoptosis) (28). In Graves’ disease only a mild lymphocytic infiltration with type 2 T helper (Th2) cell subtype predominance is observed. Th2 cells induce the production of antibodies to the receptor for TSH (TRAb), which are crucial in pathophysiology of GD. The ability of vitamin D to modulate adaptive immune system may influence the pathogenesis of autoimmune thyroid disorders.

In mice previously sensitized with porcine thyroglobulin intraperitoneal injections of calcitriol and intragastrical administration of cyclosporine A reduced severity of autoimmune thyroiditis, and, when applied together, even prevented thyroid disease (29, 30). In rats with experimental autoimmune thyroiditis, 1,25(OH)₂D₃ also prevented or ameliorated structural disruption of thyroid gland and corrected cytokine disequilibrium (31). In mouse model of Graves’ disease, persistent hyperthyroidism was observed in vitamin D deficient, but not in vitamin D sufficient animals immunized with TSH receptor. Interestingly, before immunization vitamin D deficient mice had lower T4 concentrations. These results suggest direct modulation of thyroid function by vitamin D (32).

In the last years, several clinical studies indicated an association between vitamin D deficiency and autoimmune thyroid diseases (AITD) defined as elevated antithyroid antibodies with or without characteristic ultrasonographic features (diffuse parenchymal hypoechogenicity and/or heterogeneous echogenic pattern of thyroid gland) (20, 33-38). Many authors observed that subjects with low vitamin D concentration had more frequently elevated anti-TPO antibodies (33-37, 39) and/or anti-TG antibodies (35, 36, 40, 41). In patients with AITD anti-TPO titers were highest among subjects with lowest calcidiol concentrations (42). A meta-analysis of 20 case-control studies made by Wang et al. in 2015, showed that patients with AITD have lower calcidiol levels and are more often vitamin D deficient compared to controls (43). The same results were obtained in 90 Turkish children with AITD and HT (aged 12.3 ± 2.9 years) compared to 79 age-matched healthy controls (11.9 ± 2.3 years) (44). Recently, in a group of Italian elderly subjects (168 patients, aged 81.6 ± 9.4 years) a significantly higher prevalence of AITD was observed in vitamin D deficient subjects (25(OH)D₃ < 20 ng/ml) than in those with normal calcidiol levels (28 vs. 8%, respectively, p = 0.002). In addition, in subjects with AITD a significant correlations between 25(OH)D₃ and anti-TPO antibodies (r = -0.27, p = 0.03) as well as FT3 (r = 0.35, p = 0.006), but not anti-TG antibodies, TSH or FT4 were observed (45). Interestingly, Choi et al. in a cross-sectional study on 6685 subjects observed that serum calcidiol levels were significantly lower in pre-menopausal, but not in post-menopausal women with AITD (36). It may suggest connections between vitamin D and estrogens in the development of AITD.