Numerous studies have found a link between vitamin D insufficiency and HT, and vitamin D therapy or supplementation may be indicated for HT patients [24-25]. A growing body of research suggests that vitamin D receptor (VDR) polymorphisms are linked to an increased risk of AITD [26-27]. Even if the VDR locus does not appear to have a role in HT susceptibility, vitamin D insufficiency may play a role in illness initiation by functioning as an environmental trigger. The gene discussed is VDR; the disease is hematocrit.