Low vitamin D status is associated with hypothyroid Hashimoto's thyroiditis

The association of low vitamin D status with autoimmune thyroid disease (AITD), which includes Hashimoto's thyroiditis (HT) and Graves' disease (GD), is controversial. The aim of this study was to investigate the correlation between vitamin D insufficiency and AITD, HT, and GD.

Our cross-sectional study included patients who visited a tertiary hospital for thyroid evaluation or follow-up. A total of 776 patients in whom serum 25-hydroxyvitamin D [25(OH)D], thyroid function, and anti-thyroid antibodies had been measured were analyzed retrospectively. Vitamin D insufficiency was defined as serum 25(OH)D level <75 nmol/L.

The prevalence of vitamin D insufficiency was significantly higher in the 369 patients with AITD than in the 407 without AITD (46.1% vs. 37.1%, p=0.011), and higher in the 221 patients with HT than in those with GD or non-AITD (48.9% vs. 41.9%, 37.1%, p=0.017). Among HT cases, patients with overt hypothyroidism had a significantly higher prevalence of vitamin D insufficiency (60.4% vs. 44.1%, 21.7%, 37.1%, respectively, p <0.001) and lower 25(OH)D levels (80.1 ± 47.7 vs. 99.34 ± 61.2, 110.3 ± 69.9, 99.6 ± 53.7 nmol/L, respectively, p=0.009) compared with those with euthyroidism and subclinical hypothyroidism or those without AITD. Serum 25(OH)D levels were significantly negatively correlated with serum thyroid stimulating hormone (TSH) levels after adjustment for age, sex, body mass index, and sampling season (r= -0.127, p=0.013).

Vitamin D insufficiency was associated with AITD and HT, especially overt hypothyroidism. Low serum vitamin D levels were independently associated with high serum TSH levels.