Thyroid diseases include benign and malignant diseases of the thyroid and pose a significant public health risk. Among them, TC, Autoimmune thyroid disease (AITD) and TNs are the clinically common diseases. TC is a common endocrine cancer. According to statistics, 19.3 million new cancers and 10 million deaths occurred in 2020, worldwide. TC accounted for 3.0 % and 0.4 % respectively (1). Compared with other cancers, the overall prognosis of TC is advantageous, but the financial and psychological pressure it brings to patients can not be underestimated. AITD affects 2% to 5% of the general population and includes Graves disease (GD) and Hashimoto disease (HD), common causes of hyperthyroidism and hypothyroidism, respectively (2). The prevalence of TNs ranges from 4 to 67%. Most TNs are benign, but 5% to 10% of TNs have malignant signs (3). For thyroid diseases, early identification and active treatment are crucial. It is crucial to find new risk factors and possible causal relationships in terms of prevention and treatment.

Vitamins are essential trace elements in the pathophysiological process of thyroid. Vitamin A and its derivatives, by binding to its receptors, can affect thyroid hormone (TH) signal transduction (4), regulate the effect of TH on target tissues (5), and increase the binding rate of thyroid stimulating hormone (TSH) to thyroid cells (6). B vitamins are involved in the process of oxidative stress caused by chronic inflammation, which causes increased levels of homocysteine in the blood, and hyperhomocysteinemia (Hcy) is closely associated with hypothyroidism (7). The occurrence and development of thyroid-related diseases are closely related to REDOX imbalance. Vitamins C and E have a strong ability to regulate REDOX. Studies have shown that vitamins C and E can improve oxidative damage in patients with thyroid diseases (8, 9). In 1994, Berg discovered the expression of vitamin D receptors on thyroid follicular cells in rats, suggesting that vitamin D may be involved in the pathophysiological processes of the thyroid (10). Experimental studies in rats suggested that vitamin D may have central and peripheral effects on the release of TSH and TH. Currently, clinical trials of vitamin D supplementation for AITD and TC have been conducted, but the results have been inconsistent (11).

In order to further uncover the correlation between circulating vitamin levels and thyroid diseases, more rigorous studies are needed. However, traditional observational studies are often affected by confounding factors and reverse causality, and there are certain limitations in the reliability of results. Therefore, we conducted an MR study of multiple circulating vitamin levels(vitamin A, B₉, B₁₂, C, D, and E) and thyroid diseases(autoimmune hyperthyroidism, autoimmune hypothyroidism, TNs, and TC). Unlike traditional studies, MR studies genetically explain the cause-and-effect relationships between circulating vitamin levels and thyroid diseases. In MR studies, because parents’ alleles are randomly assigned at conception, genetic variation precedes disease development and is not influenced by environmental confounders. Therefore, MR studies avoid confounding bias and reverse causality, and the research results are more robust and reliable (12).

In this study, we performed the bidirectional causal relationship between circulating vitamin levels and thyroid diseases using a bidirectional two-sample MR Study. Genetic evidence suggested that circulating vitamin C level has a protective effect on autoimmune hypothyroidism. In reverse MR, The results showed that reduced vitamin A levels are related to the risk of developing autoimmune hyperthyroidism, and vitamin D increase can contribute to development of TNs. In addition, there was no significant association between circulating vitamin levels and thyroid diseases.

Vitamin C is a member of the body’s non-enzymatic antioxidant system. It is a cofactor of many biosynthetic and gene regulatory enzymes, and plays an important role in a variety of immune regulation, chromatin remodeling, and cell division (23). At the physiological concentration of human plasma vitamin C (40-80μM), it can effectively remove reactive oxygen species (ROS) and act as an antioxidant; at high doses (10-20 mM), vitamin C can induce oxidative stress and inhibit tumor growth without significant damage to normal cells and tissues (23). An animal study demonstrated a protective effect of vitamin C against boldenone undecylenate-induced autoimmune hypothyroidism of Wistar rats (24). Karimi F (25) randomly divided 100 patients with autoimmune thyroiditis into the Selenium (Se) treatment group, vitamin C treatment group, and placebo treatment group. antithyroid peroxidase antibody (TPO-Ab) concentration decreased in both Se and vitamin C treated groups, but no change was observed in the placebo group. The above studies are consistent with the results of this study. The pathogenesis of autoimmune hypothyroidism is related to oxidative stress (26) and abnormal immune system (27). Vitamin C may affect gene expression of oxidative stress and immune system, and has a protective effect on autoimmune hypothyroidism. Although evidence of large-scale evidence-based medicine is still lacking, this study provides new ideas for improving our understanding of the pathogenesis and treatment of autoimmune hypothyroidism.

Studies (28) have also shown that high doses of vitamin C can affect the REDOX equilibrium and cell metabolism of cancer cells by mediating the pro-oxidation mechanism, thereby inhibiting the growth of TC cells. However, this MR study did not observe a direct causal relationship between vitamin C and TC, which may be related to the small number of cases of TC, or may have an anti-tumor effect through indirect effects, but no matter which possibility, it is worth further study.

On the other hand, the results of this study showed that autoimmune hyperthyroidism is a risk factor for reduced vitamin A levels. Vitamin A is able to regulate thyroid homeostasis alone or interact with other micronutrients, especially with iodine (29). And vitamin A also has antioxidant effects, and the pathogenesis of autoimmune hyperthyroidism is related to impaired oxidative stress. The decrease in vitamin A level may be due to its participation in the process of regulating oxidative stress damage. A study has shown that vitamin A can alleviate the clinical symptoms of patients with hyperthyroidism and reduce the metabolic rate (30). More and more studies have also shown that the levels of vitamin A and its derivatives can regulate TH from gene expression levels and their effects on target tissues (31). However, whether vitamin A supplementation can make hyperthyroidism gain clinical benefits and possible channels of action are worth looking forward to in future large-scale randomized controlled studies.

Vitamin D₃ is one of the most important types of vitamin D in the body. Calcitriol (the active form of vitamin D3) can bind to the vitamin D receptor (VDR). VDR is involved in regulating the expression of more than 1000 genes, including thyroid tissue (32–34). More and more studies have shown that vitamin D deficiency can cause thyroid dysfunction and autoimmune thyroid disease (11), and vitamin D supplementation can improve thyroid function and treat autoimmune thyroid disease (35, 36). However, vitamin D supplementation did not reduce the overall risk of hypothyroidism (37); It also had poor therapeutic effect on GD patients (38). Another study (39) suggested that vitamin D deficiency was a risk factor for TC and that vitamin D supplementation inhibited the proliferation (40) and ability of TC cells to metastasize (41) and reduced the risk of advanced cancer in individuals who were undiagnosed at the start of the study (42). However, a study (43) has shown no link between vitamin D levels and the risk of TC. The direct causal relationship between TC and autoimmune thyroid disease and vitamin D has not been observed in our MR study. This can also explain the contradiction of the current clinical research results. There were few studies on the relationship between TNs and vitamin D. Jinzhuo Fan found a negative correlation between TNs and vitamin D levels in a study of 875 centenarians in Hainan Province, China (44). A study in Turkey also found that the level of vitamin D was low in patients with TNs (45). This is the opposite of our study. The MR Study was independent of environmental confounding factors, but we included only European populations. Turkey straddles Asia and Europe, and China belongs to Asia. Therefore, the inconsistency of the results may be due to the different populations.Therefore, what is the optimal level of vitamin D maintenance in patients with thyroid diseases and can benefit from the protective effect of vitamin D needs to be further determined by large sample and multi-center clinical studies.

Vitamin E can play an antioxidant role by acting as a structural component of biofilms as well as cleaning up free radicals (46). But the role of vitamin E in thyroid diseases has not been fully appreciated. Only a few studies have shown that vitamin E reduces the aggressiveness of TC in patients (47) and reduces oxidative damage caused by excessive TH in the blood circulation of hyperthyroid animals (9). However, our MR Study did not show any statistically significant direct cause-and-effect relationship between vitamin E and thyroid diseases. The vitamin B family is involved in folate and homocysteine metabolism in different ways. Deficiencies in folic acid, vitamins B₉, and B₁₂ can cause increased levels of homocysteine in the blood (48, 49), but Hcy is associated with impaired TH sensitivity in adults with normal thyroid function (50). No studies have shown a direct association between vitamin B₉ and B₁₂ and thyroid diseases, which is consistent with our findings.

There are some limitations to this study. Firstly, although our inclusion of only European populations avoids the bias caused by racial stratification, it also limits the possibility of generalizing our study to the entire population. Secondly, to have sufficient IVs in MR Analysis, we weakened the independence of SNPs (p < 5E-06), but the F statistics of SNPs are all greater than 10, and they meet the conditions of MR Analysis. Thirdly, although we use the latest statistical data with the largest sample size, the small sample size still limits the reliability of MR analysis. Nevertheless, our study is the first to elucidate the association between circulating vitamins and thyroid diseases at the genetic level, providing new insights into the relationship between the two and providing valuable information for the study of the pathogenesis of thyroid diseases.

Taken together, this study reveals a bidirectional causal relationship between circulating vitamin levels and thyroid diseases, providing new insights and evidence for the etiology, screening, and management of thyroid diseases and clinical micronutrient deficiencies. Further research is needed to elucidate the underlying mechanisms between vitamins and thyroid diseases and to verify this association through basic experiments as well as large-scale randomized controlled trials.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

WZ: Data curation, Writing – original draft, Methodology. EL: Data curation, Visualization, Writing – original draft. HQ: Writing – review & editing, Conceptualization, Data curation, Methodology.