1 WHAT IS KNOWN

Hashimoto's thyroiditis (HT), also called chronic lymphocytic thyroiditis, is the most prevalent organ-specific autoimmune disorder as well as the most common cause of thyroid hypofunction, with an incidence of 0.3–1.5 cases per 1000 people in developed countries.1, 2 This autoimmune event is believed to be initiated by the activation of thyroid antigen-specific auxiliary T lymphocytes and patients may present with various thyroid function states but most of them eventually evolve into hypothyroidism.3 The main purpose of HT treatment is the control of hypothyroidism, including oral administration of a synthetic hormone to achieve normal circulating thyrotropin levels. But the effectiveness of the current treatments such as glucocorticoids, levothyroxine and specific diet for the management of HT has been questioned in recent years.4-7

Vitamin D is a steroid molecule whose primary function is to control bone metabolism and calcium and phosphorus homeostasis. Recent evidence suggested that vitamin D is also associated with non-skeletal roles including in autoimmune diseases, infectious diseases, metabolic syndrome, cardiovascular diseases and cancers.8-10 A recent published meta-analysis analysed the effects of vitamin D11 on autoantibodies in patients with autoimmune thyroiditis (AIT). Based on the combined results of six randomized controlled trials, they concluded that vitamin D treatment might be a good choice for AIT based on vitamin D, especially for those with vitamin D deficiency, which is common worldwide.12 Low levels of vitamin D were found to be associated with HT, and the association between vitamin D deficiency, HT pathogenesis and thyroid hypofunction has been demonstrated in several studies.13-15 Given the low cost and the minimal side effects of oral vitamin D supplementation, the treatment of vitamin D supplementation in patients with HT may be recommended.

1.1 Objective

In light of the current information, vitamin D might be effective in the treatment of hypothyroidism in patients with HT. Therefore, this study aimed to review the association between vitamin D treatment in patients with HT by assessing patients’ serum circulating 25-hydroxyvitamin D level to evaluate whether a change occurs in the course of disease.

2 METHODS 2.1 Ethical statement

We developed the framework of the current systematic review and meta-analysis according to the recommendations issued by the Cochrane Collaboration for the purpose of ensuring the methodological quality because we did not register formal protocol.16 We did not impose ethical approval and patients’ informed consent because all essential data in the current systematic review and meta-analysis were extracted from published studies.

2.2 Literature search

This meta-analysis was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.17 The relevant clinical trials were searched based on the PICO process.18 A systematic search was performed from PubMed, Embase and the Cochrane library for available studies published up to August 2021, using the MeSH term ‘Hashimoto Disease’ and ‘Vitamin D’, as well as relevant keywords. For studies that have not been completed and published but had previously published their design and protocol with their registration number in ClinicalTrials.gov, we manually searched them to ensure whether the results were posted or not.

2.3 Eligibility criteria

The eligibility criteria were as follows: (1) population: patients diagnosed with HT; (2) interventions: treated by vitamin D; (3) control: placebo or matched individualized therapy; (4) study type: any prospective studies or RCTs published in scientific peer-reviewed journals; (5) outcome: serum 25-hydroxyvitamin D level after treatment; and (6) language was limited to English. No ethical consent was required because this study was performed based on previous data.

2.4 Data extraction

Two investigators (Hui Jiang and Xiao-luo Chen) independently extracted the following items using the pre-designed data extraction sheet: basic characteristics of the study including first author, year of publication, country, and type of study design, patients’ characteristics including sample size, mean age and gender, and clinical characteristics of study including vitamin D status at inclusion, serum thyrotropin levels at inclusion, treatment in both groups, dose of treatment, follow-up period. Serum 25-hydroxyvitamin D level after treatment was included as primary outcome. If an included study was designed to have more than two groups, then the methods recommended by the Cochrane Handbook for Systematic Reviews of Interventions were used to divide the individual study into two unique RCTs or combine groups to create a single pair-wise comparison.19 If essential information was missed from the original study, then the leading author was contacted for additional information. Any inconsistencies in data extraction were solved based on the consensus principle.

2.5 Outcomes

The primary outcome was the serum 25-hydroxyvitamin D level after treatments. The secondary outcomes were the levels of thyroid peroxidase antibodies (TPO-Ab), thyroglobulin antibodies (TG-Ab), thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4) after treatments.

2.6 Quality of the evidence

The level of evidence of all included studies was assessed independently by two authors (Hui Jiang and Xiao-luo Chen) using the RoB-2 criteria or MINORS (Methodological Index for Non-Randomized Studies) scoring system.20, 21 Discrepancies in the assessment were resolved through discussion until a consensus was reached.

2.7 Statistical analysis

All analyses were performed using STATA SE 14.0 (StataCorp). The outcomes were presented as weighted mean differences (WMD). The effects and corresponding 95% confidence intervals (CIs) were used to compare the outcomes. For studies that did not present their results as means ±standard deviations, the results were estimated based on the reported parameters (median, standard error, IQR or 95% CI).22 Statistical heterogeneity among studies was calculated using Cochran's Q test and the I2 index. An I2 > 50% and a Q test p < 0.10 indicated high heterogeneity, and the random effects model was used; otherwise, the fixed-effects model was applied. p-values <0.05 were considered statistically different. Sensitivity analysis was performed using the leave-one-out method.20 We did not assess the potential publication bias by funnel plots and Egger's test, because the number of studies included in every meta-analysis was fewer than ten, in which case the funnel plots and Egger's test could yield misleading results and are not recommended.

3 RESULTS 3.1 Study inclusion

Figure 1 presents the study inclusion process. A total of 416 studies were first retrieved, and 61 trials were left after removing the duplicates. Then, 182 were excluded because of the type of article or the language. From the 173 trials left, after reviewing the full texts, 56 case-report studies, 36 retrospective designed studies, 11 in vivo studies, 29 studies with unsuitable population, 34 with inappropriate interventions and 1 with unwanted outcome were excluded. Therefore, a total of seven cohorts of patients from six studies (3 prospective cohort study and 3 RCTs) entered our analysis (Table 1).23-28 At the end of our research, 258 patients with HT were included in our analyses.

PRISMA 2009 flow diagram

TABLE 1. Characteristics of the literatures Author, year Country Study design VD status Serum thyrotropin levels, mU/l Intervention Control Sample size Dose Gender Age (intervention/control), y Follow-up Intervention Control Intervention Control Chahardoli, 2019 Iran RCT Not limited NA VD Placebo 19 21 50000 IU/week / Female 36.4 ± 5.2/35.9 ± 7.8 3 months Nodehi, 2019 Iran RCT Not limited NA VD Placebo 17 17 50000 IU/week / Female 36.4 ± 5.2/35.9 ± 7.8 3 months Vahabi, 2017 Iran RCT Insufficiency NA VD Placebo 30 26 50000 IU/week / Both 43.6 ± 1.6/44.1 ± 1.6 3 months Krysiak, 2016 Poland Prospective Normal 0.4 to 8 VD + Levothyrotxine Levothyroxine 16 18 2000 IU/d Levothyroxine: NA NA Female 34 ± 7/36 ± 6 6 months Krysiak, 2017a Poland Prospective Insufficiency 0.45 to 4.5 VD + Simvastatin Simvastatin 19 9 VD: 2000 IU/d Simvastatin: 40 mg/day 40 mg/day Female 38 ± 7/37 ± 6 6 months Krysiak, 2017b Poland Prospective Insufficiency 0.45 to 4.5 VD Simvastatin 20 9 2000 IU/day 40 mg/day Female 39 ± 5/37 ± 6 6 months Krysiak, 2018 Poland Prospective Not limited 0.4 to 4 VD Selenomethionine 20 17 4000 IU/day 200 µg/day Male 35 ± 8/34 ± 7 6 months Abbreviations NA, not applicable; RCT, randomized control trial; VD, vitamin D. 3.2 Quality of all included studies

Among the three randomized controlled trials,23, 27, 28 all three studies were graded as low risk of bias in the assessment for bias arising from the randomization process, bias due to missing outcome data and bias in selection of the reported result. Some concerns were raised in the assessment for bias due to deviations from intended interventions in one study.27 All three studies were concerned in the assessment for bias in measurement of the outcome. Among the four prospective studies,24-26 the total quality score of individual study was all more than 6. We summarized the results of appraising quality of all included studies in Online appendix A1.

3.3 Effect of vitamin D on the serum 25-hydroxyvitamin D

All 7 cohorts of patients from six studies reported the serum 25-hydroxyvitamin D level after treatment in both treatment and control group. Significant difference was found (WMD = 19.00, 95% CI: 12.43, 25.58, p < 0.001; I2 = 90.0%, pheterogeneity < 0.001, Figure 2A) between the vitamin D group and control group in serum 25-hydroxyvitamin D level. The sensitivity analyses showed that no specific study contributed to heterogeneity (Online appendix A2a).

Forest plot comparing the autoimmunity markers between vitamin D group with the control group. (A) 25-Hydroxyvitamin D Level; (B) thyroid peroxidase antibodies; (C) thyroglobulin antibodies; (D) thyroid-stimulating hormone; (E) free thyroxine; (F) free triiodothyronine

3.4 Secondary outcomes

Six cohorts of patients from five studies reported the level of TPO-Ab after treatment from vitamin D or control.23-26, 28 Combined results indicated vitamin D significantly reduced the level of TPO-Ab compared to the control group (WMD = −158.18, 95% CI: −301.92, −14.45, p = 0.031; I2 = 68.8%, pheterogeneity = 0.007, Figure 2B). The sensitivity analysis showed that no specific study contributed to heterogeneity (Online appendix A2b).

Five cohorts of patients from four studies reported the level of TG-Ab after treatment from vitamin D or control.23-26 Combined results indicated vitamin D did not significantly reduce the level of TG-Ab compared to the control group (WMD = −68.21, 95% CI: −143.04, 6.62, p = 0.074; I2 = 35.8%, pheterogeneity = 0.183, Figure 2C). The sensitivity analysis showed that no specific study contributed to heterogeneity (Online appendix A2c).

Combined results showed vitamin D did not significantly change the levels of TSH, FT3 and FT4 compared to the control group (Figure 2D–F).

3.5 Subgroup analyses

All subgroup analyses were presented in Table 2. The level of serum 25-hydroxyvitamin D was increased from the treatment of vitamin D regardless of the length of follow-up, the vitamin status at initiation and the gender of patients compared to the controls group in patients with HT. The sensitivity analyses showed that no specific study contributed to heterogeneity Online appendix A2d&e&f).

TABLE 2. Combined results of the subgroup analyses N WMD (95% CI) p (Heterogeneity) I2, % p 25-hydroxyvitamin D 7 19.00 (12.43, 25.58) <0.001 90 <0.001 Follow-up 3 months 3 29.89 (26.22, 33.55) 0.482 0 <0.001 6 months 4 13.62 (11.49, 15.74) 0.582 0 <0.001 Vitamin D status Insufficient 3 18.80 (8.05, 29.56) <0.001 96.1 0.001 Normal 1 17.00 (9.60, 24.40) . . <0.001 Both 3 20.25 (8.58, 31.92) 0.023 73.4 0.001 Gender Women 5 16.60 (11.43, 21.77) 0.006 72 <0.001 Men 1 15.00 (9.47, 20.53) . . <0.001 Both 1 30.60 (26.43, 34.77) . . <0.001 Thyroid peroxidase antibodies 6 −158.18 (−301.92, −14.45) 0.007 68.8 0.031 Follow-up 3 months 2 −61.05 (−127.69, 5.59) 0.727 0 0.073 6 months 4 −251.51 (−520.68, 17.66) 0.003 78.4 0.067 Vitamin D status Insufficient 3 −201.15 (−501.79, 99.49) 0.061 64.3 0.190 Normal 1 −455.00 (−718.29, −191.71) 0.001 Both 2 −53.58 (−115.20, 8.05) 0.513 0 0.088 Gender Women 4 −260.38 (−509.17, −11.60) 0.002 79.2 0.040 Men 1 −11.00 (−152.75, 130.75) . . 0.879 Both 1 −16.00 (−309.42, 277.42) . . 0.915 Thyroglobulin antibodies 5 −68.21 (−143.04, 6.62) 0.183 35.8 0.074 Follow-up 3 months 1 −36.50 (−130.06, 57.06) . . 0.445 6 months 4 −124.48 (−249.12, 0.16) 0.171 40.1 0.050 Vitamin D status Insufficient 2 −285.63 (−555.97, −15.29) 0.491 0 0.038 Normal 1 −278.00 (−548.15, −7.85) . . 0.044 Both 2 −29.53 (−110.85, 51.79) 0.768 0 0.477 Gender Women 4 −83.94 (−167.97, 0.09) 0.134 46.2 0.050 Men 1 −8.00 (−172.43, 156.43) 0.924 Thyrotropin 6 −0.00 (−0.66, 0.66) <0.001 88.9 0.998 Follow-up 3 months 2 0.20 (−1.92, 2.31) <0.001 93.5 0.855 6 months 4 −0.09 (−0.36, 0.18) 0.675 0 0.503 Vitamin D status Insufficient 3 0.30 (−0.73, 1.34) <0.001 93.5 0.569 Normal 1 −0.40 (−1.27, 0.47) . . 0.366 Both 2 −0.05 (−0.44, 0.34) 0.066 70.3 0.808 Gender Women 4 −0.29 (−0.62, 0.03) 0.594 0 0.079 Both 1 1.22 (0.89, 1.55) <0.001 Men 1 0.10 (−0.32, 0.52) 0.640 Free thyroxine 5 0.09 (−0.51, 0.70) 0.654 0 0.760 Follow-up 3 months 1 −0.40 (−1.36, 0.56) . . 0.416 6 months 4 0.43 (−0.36, 1.21) 0.860 0 0.290 Vitamin D status Insufficient 2 0.20 (−0.89, 1.28) 0.856 0 0.723 Normal 1 1.00 (−0.55, 2.55) . . 0.206 Both 2 −0.23 (−1.07, 0.61) 0.484 0 0.589 Gender Women 4 0.06 (−0.59, 0.72) 0.497 0 0.845 Men 1 0.30 (−1.41, 2.01) . . 0.731 Free triiodothyronine 5 0.03 (−0.13, 0.19) 0.337 12 0.707 Follow-up 3 months 1 −0.03 (−0.24, 0.18)