Association between vitamin D deficiency and risk of thyroid cancer: a case-control study and a meta-analysis.

Purpose: Although vitamin D is reportedly associated with various cancers, the association between vitamin D and thyroid cancer is indefinite. We aimed to investigate whether this association applies to thyroid cancer (TC).
Methods: A total of 276 Chinese Han people were recruited in a current matched case-control study. Multivariable conditional logistic regression was computed to estimate the association between plasma 25(OH)D and papillary thyroid cancer (PTC). In addition, we searched relevant studies in PubMed and Web of Science databases before December 2017 to conduct a meta-analysis.
Results: In our case-control study, plasma 25(OH)D concentration was inversely associated with PTC risk (highest tertile vs lowest tertile: adjusted OR = 0.25; 95% CI 0.10, 0.61; P _trend  = 0.003). This association was independent of body mass index and physical activity (all adjusted P _interaction  > 0.05). A total of 11 studies were included in the meta-analysis, among which ten studies have been published and one was our case-control study. Compared with 25(OH)D non-deficient group, the pooled OR of TC was 1.42 (95% CI 1.17, 1.73) in the deficient group. Similarly, blood 25(OH)D levels in patients with TC were tend to be lower than those in the controls (SMD = - 0.20, 95% CI - 0.36, - 0.03).
Conclusions: A high level of circulating 25(OH)D was associated with a decreased TC risk. This association has important significance in public health and should, therefore, be further studied.