Vitamin D status in Hashimoto’s thyroiditis and its association with vitamin D receptor genetic variants

•VDR gene FokI AA (rs2228570) is associated with Hashimoto's thyroiditis.•VDR gene FokI AA (rs2228570) variant could have normal levels of 25−OH-vitamin D3.•Thyroid volume of cutoff ⋟ 4.8 ml can help in diagnosis of Hashimoto's thyroiditis. Hashimoto's thyroiditis (HT) is considered the predominant cause of hypothyroidism in iodine sufficient countries. The deficiency of 25−OH-vitamin D3 serum level and the variation of vitamin D receptor (VDR) gene were implicated in a number of autoimmune disorders. This study aimed to test the hypothesis linking between VDR FokI and BsmI variants and HT, in addition to explain their impact on 25−OH-vitamin D3 serum level. Cross sectional study included 160 hypothyroid subjects, 112 patients with HT and 48 hypothyroid non-HT controls. They were diagnosed based on anti-TPO Ab and or anti-TG Ab results. All cases were subjected to full history taking, thyroid ultrasound examination and a panel of assays (TSH, f.T3, f.T4, anti-TPO Ab, anti-TG Ab, calcium, alkaline phosphatase and phosphate). Serum 25−OH-vitamin D3 was assayed using HPLC-UV method. VDR variants (FokI and BsmI) were genotyped using real-time PCR. FokI AA genotype was statistically higher in HT patients than control group (P value = 0.02) with subsequently higher serum 25−OH-vitamin D3 level in comparison to all other genotypes (P value = 0.039). Serum 25−OH-vitamin D3 level was statistically indifferent between HT and control group (P value = 0.223). A statistically significant increase in total thyroid volume was observed in HT group (P value = 0.002). FokI AA genotype is more associated with HT in Egyptian patients compared to hypothyroid non-HT controls. Moreover, patients with FokI AA genotype have statistically higher levels of 25−OH-vitamin D3 suggesting VDR dysfunction even in patients expressing normal level of vitamin D.