Vitamin D and thyroid disorders: a systematic review and Meta-analysis of observational studies

Background The contribution of vitamin D to thyroid disorders has received paramount attention; however, results are mixed. Hence, we designed a systematic review and meta-analysis to obtain a definitive conclusion. Methods The search included PubMed, ISI Web of Science, Scopus, and Google Scholar databases up to March 2021 to collect available papers reporting the relationship between serum levels of vitamin D and thyroid disorders. The pooled effect was reported as weighted mean difference (WMD) and 95% confidence interval (CI). Results Out of 6123 datasets, 42 were eligible to get into this systematic review and meta-analysis. Serum vitamin D was markedly lower in autoimmune thyroid diseases (AITD) (WMD − 3.1 ng/dl; 95% CI, − 5.57 to − 0.66; P = 0.013; I2 = 99.9%), Hashimoto’s thyroiditis (HT) (WMD − 6.05 ng/dl; 95% CI, − 8.35 to − 3.75; P < 0.001; I2 = 91.0%) and hypothyroidism patients (WMD − 13.43 ng/dl; 95% CI, − 26.04 to − 0.81; P = 0.03; I2 = 99.5%), but not in subjects with Graves’ disease (GD) (WMD − 4.14 ng/dl; 95% CI, − 8.46 to 0.17; P = 0.06; I2 = 97.5%). Conclusions Our findings suggested lower vitamin D levels in patients with hypothyroidism, AITD, and HT compared to healthy subjects. However, the link between serum vitamin D and GD was only significant among subjects ≥40 years old.


Background
Vitamin D is an essential fat-soluble nutrient with hormone-like activity [1,2]. Vitamin D deficiency is among the most common health problems in the world in all age groups, even in low latitude countries with sufficient UV radiation or industrialized countries with a long history of vitamin D fortification strategies. Previous studies have indicated that about one billion people around the world have been diagnosed with vitamin D deficiency [3]. Recent evidence has shown the prevalence of vitamin D deficiency in both developed and developing countries, which in Europe is 13% and in the United States 19% [4][5][6]. Vitamin D deficiency has increased in recent years in Iran, and reports have revealed that the vitamin D deficiency rate is nearly 50% [2]. Risk factors for vitamin D deficiency include aging, female sex, winter season, obesity, malnutrition, lack of sun exposure, and dark skin pigmentation [7]. Accordingly, sufficient attention to vitamin D deficiency diagnosis is needed for treatment to decrease its undesirable effects on human health.
Over the last decades, numerous studies have indicated that low serum vitamin D levels are associated with a series of diseases such as high blood pressure, heart disease, diabetes, cancer, mood disorders, multiple sclerosis, and autoimmune diseases [8,9]. This vitamin exerts its biological actions through nuclear vitamin D receptors (VDR) that reside in most human cells and tissues [10]. Hence, vitamin D plays its role through the regulation of gene expression in places where its receptors exist, such as the endocrine system [11].
The thyroid gland is one of the largest endocrine glands with many roles for homeostatic control, including growth, energy expenditure, and metabolism [12,13]. Any thyroid disorder could result in a cluster of metabolic ailments [12][13][14]. Recent literature has illustrated the presumed association between serum vitamin D and thyroid diseases [15]. This association results from the presence of similar receptors for vitamin D and thyroid hormones, named steroid or nuclear hormone receptors. These shreds of evidence show the importance of the role of vitamin D in thyroid function and the association between vitamin D deficiency and thyroid diseases. However, some studies observed no significant relationship [16][17][18]. Therefore, we conducted a systematic review and meta-analysis to survey all observational studies regarding the association between serum vitamin D levels and thyroid disorders, including hypothyroidism, autoimmune thyroid disease (AITD), Hashimoto's thyroiditis (HT), and Graves' disease (GD) among the adult population.

Methods
After registration on the Prospero database (CRD42020187237), this dataset was conceived pursuant to the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) statement [19].

Search strategy
We performed a systematic literature search up to March 2021 in four databases, including PubMed, ISI Web of Science, Scopus, and Google Scholar to find papers reporting the relationship between serum levels of vitamin D and thyroid disorders. The terms used for the database search were "vitamin D" OR "25-hydroxyvitamin D" OR "1,25-dihydroxyvitamin D" OR "cholecalciferol" OR "ergocalciferol" OR "calcitriol" OR "vitamin D 3 " AND "hypothyroidism" OR "hypothyroid" OR "thyroid disorder" OR "triiodothyronine" OR "thyroxin" OR "thyroid stimulating hormone" OR "thyroid hormones" OR "hyperthyroidism" OR "hyperthyroid" OR "autoimmune thyroiditis" OR "anti-thyroid peroxidase" OR "Graves' , disease", OR "Hashimotoʼs thyroiditis". The authors also screened the reference sections of the contained articles to spot any other eligible studies not captured via the online database search.

Study selection
Before the screening process, all search results were exported into the EndNote X7 software (Thomson Corporation, Stamford, USA). Original observational human studies (case-control or cross-sectional) about the association between serum vitamin D and thyroid disorders with reported means, medians, or odds ratios (ORs), and the corresponding 95% confidence intervals (CIs) were included. Studies that recruited women during pregnancy or lactation, non-human datasets, reviews, case reports, editorials, poster abstracts, non-original, or irrelevant datasets were excluded. Two investigators were responsible for the selection process to lower the potential error (S. T & A. A). The difference of opinion was dealt with the cooperation of a third reviewer (G. A).

Data extraction
The data of interest were extracted from each study using a predefined excel form in a duplicate and blinded manner. For each study, the extracted information was as follows: first author's name, publication year, location, sample size, sex, age, body mass index (BMI), study design, vitamin D assay method, the season of sample collection, and population health status. Two independent reviewers conducted the processes of data extraction (S. T & A. A). The difference of opinion on a given study was dealt with via discussion, and if necessary, arbitration by a third reviewer (G. A).

Statistical analysis
Authors run a meta-analysis, including weighted mean difference (WMD) with 95% CIs plus a random effect model, to estimate the quantitative summary of the association between vitamin D and thyroid disorders, including hypothyroidism, AITD, HT, and GD. Mean ± standard deviation (SD) was extracted to estimate the pooled effects using WMD. For datasets with a standard error (SE), SD was determined using the following equation: SE × √n. Related equations were used to obtain the preferred data out of the median ± range or interquartile range [22]. If a study presented analyses stratified by certain key variables such as participants' health status, stratified estimates were assumed to be independent of each other and included as a separate unit of observation in the meta-analysis. To assess the influence of each study on the stability of the meta-analysis results, the authors carried out a sensitivity analysis. Moreover, subgroup analysis based on age, gender, geographical population, study design, and population health status was executed whenever it was possible. STATA software, version 11.2 (Stata Corp, College Station, TX, USA) was used for data analysis with P-values < 0.05 considered as statistically significant. Heterogeneity was checked using the I 2 index. I 2 index equal to 25, 50, or 75% represents low, moderate, and high heterogeneity, respectively [23]. The funnel plots were visually inspected to detect any publication bias, and Egger's and Begg's tests defined the extent of funnel plot asymmetry [24].

Search results
Our initial search through databases identified a total of 6123 papers. With duplicates removed, the 3062 remaining articles were examined based on the review of the titles and abstracts by two independent reviewers. Authors retrieved and reviewed 98 articles based on the full text, and finally, 35 studies were eligible to get into the current study. Moreover, the works of Bouillon et al. [25], Unal et al. [26], Ma et al. [27], Ke et al. [28], Fawzy et al. [29], Toulis et al. [30], and Komisarenko et al. [31] were divided into two different studies. Therefore, a total of 42 eligible studies were included in the meta-analysis. The PRISMA flowchart briefly states the results of the study selection process (Fig. 1).

Discussion
As far as we know, this is the first comprehensive attempt to examine the association between serum levels of vitamin D and thyroid disorders in form of a systematic review and meta-analysis. It revealed that vitamin D was significantly associated with hypothyroidism, AITD, and HT. Furthermore, vitamin D was significantly lower among ≥40-year-old GD subjects. Some issues should be considered when elucidating the results. First of all, there were substantial inter-assay differences in the performance of commercially available kits for serum vitamin D assay [63]. This notion may impact the results and play a considerable role as a heterogeneity factor. Also, seasonal variation in serum vitamin D should be  [64]. This fact has been excused in some papers [25,26,31,34,35,37,46,48,50,56]. Inconsistencies in terms of the season of sample collection could also influence our final results. Besides, it has been proven that the bioavailability of vitamin D diminishes among overweight/obese individuals [65]; however, most of the included articles disregarded BMI as a confounding factor and probable source of heterogeneity [17, 18, 25, 26, 29, 32, 34, 37-39, 41-44, 46, 48, 49, 54]. Pursuant to previous reports, the role of vitamin D in immunity is through producing anti-inflammatory and immune-regulatory markers via VDR expression in the nucleus of cells [66]. VDR is involved in cellular immunity function by stimulating innate and adaptive immune responses [66]. Polymorphisms of the VDR are related to the predisposition of people to thyroid disorders such as hypothyroidism [12,67]. VDR modulates the effects of vitamin D as its specific and intracellular receptor. Polymorphisms of the VDR gene may decrease vitamin D activity [12]. Through a meta-analysis, Wang et al., reported a significant link between VDR gene polymorphisms and autoimmune thyroid disorders in diverse ethnic groups [66]. Vitamin D has hindering effects on the production of inflammatory cytokines such as interleukin (IL)-1, IL-6, IL-8, IL-12, and tumor necrosis factor (TNF)-α [68,69]. Also, it suppresses dendritic cell differentiation and maturation via a reduced expression of the major histocompatibility complex (MHC) class II molecules, co-stimulatory molecules, and IL-12 [68,69]. Additionally, vitamin D makes the induction of T regulatory cells simpler to diminish T cell-dependent immune responses in autoimmune diseases [68,69]. T and Bcells reaction to thyroid antigens in geneticallyvulnerable subjects lead to HT [42]. Based on previous studies, serum levels of 25-OH D3 < 20 ng/mL can exacerbate positive thyroid autoantibodies such as antithyroid peroxidase (TPOAb) and anti-thyroglobulin (TgAb) [70][71][72]. Additionally, high serum levels of calcium and phosphorous and low levels of circulating parathyroid hormone may inhibit renal 25(OH) D1-α hydroxylase function [17]. Thus, due to lower production of 1,25 (OH)2 D, serum 24,25 (OH) D and thyroid hormone levels will be higher [17]. Overall, vitamin D could contribute to the prevention or correction of hypothyroidism and improvement of thyroid function [17]. The limitations of this study are as follows. There was significant heterogeneity in our study that may have affected the results and diminished the generalizability of the outcomes. The probable sources of heterogeneity might be differences in age, gender, BMI, study design, vitamin D assay methods and kits, the season of sample collection, geographical variation, and quality of the studies. Moreover, the nature of cross-sectional studies makes it impossible to draw a causal link between variables. Furthermore, there was no information regarding dietary vitamin D intake that could have affected serum levels of vitamin D.

Conclusion
In the present systematic review and meta-analysis, vitamin D levels were significantly lower in hypothyroidism, AITD, and HT patients compared to healthy people. However, there was no significant association between serum vitamin D and GD, except among subjects ≥40 years old. The effect sizes of the included studies showed significant heterogeneity which might affect the interpretation of results. Further well-designed prospective cohort studies and clinical trials are needed for a better understanding of the relationship between vitamin D and thyroid disorders.  reporting items for systematic reviews and meta-analysis; OR: Odds ratio; CI: Confidence intervals; BMI: Body mass index; WMD: Weighted mean difference; SD: Standard deviation; SE: Standard error; IL: Interleukin; TNF: Tumor necrosis factor; MHC: Major Histocompatibility Complex; TPOAb: Anti-thyroid peroxidase Antibody; TgAb: Anti-thyroglobine Antibody