Vitamin D deficiency as a risk factor for thyroid cancer: A meta-analysis of case-control studies

The association between vitamin D deficiency and thyroid cancer is controversial. Some studies have demonstrated that higher serum vitamin D levels might protect against thyroid cancer, whereas others have not, or have even indicated the opposite to be the case. The aim of this meta-analysis was to investigate the association between vitamin D deficicency and thyroid cancer and propose that vitamin D deficiency is a risk factor for thyroid cancer. This was a meta-analysis of 14 articles of the association between vitamin D deficiency and thyroid cancer. Databases including PubMed, Cochrane library, Sinomed, CNKI, Wanfang, and clinical trial register centers, were searched for case-control studies of vitamin D in thyroid cancer. Fourteen studies were included in this meta-analysis. A fixed-effect model was used to merge the standardized mean difference value of serum 25-hydroxyvitamin D levels. The pooled effect showed that the levels of serum 25-hydroxyvitamin D were lower in patients with thyroid cancer preoperatively than in the controls (–0.22; 95% confidence interval [CI], –0.36 to –0.09; P = 0.001). There was no difference after thyroid cancer patients underwent thyroidectomy (–0.19; 95% CI, –0.47 to 0.10; P = 0.21). A fixed-effect model was used to pool the odds ratio of thyroid cancer and vitamin D deficiency. It showed that the pooled odds ratio from six studies was 1.30 (95% CI, 1.00–1.69; P = 0.05). Subgroup analysis of 25-hydroxyvitamin D levels between different pathologic characteristics in patients with thyroid cancer was summarized, but no statistical differences were determined. Lower serum 25-hydroxyvitamin D levels were associated with increased risk for thyroid cancer. On the other hand, vitamin D deficiency may act as a risk factor for thyroid cancer.