VDR and hematocrit: The aim of this section is to present: (1) factors that may influence VDR expression, (2) the diversity of studies, populations, and methodologies assessing the relationship between VDR gene polymorphisms and vitamin D levels in the context of HT and other various diseases, and (3) to demonstrate the association between specific types of polymorphisms and the severity of HT and the response to vitamin D supplementation.