PTH and vitamin D deficiency: The current UK definition of vitamin D deficiency (less than 25 nmol l−1) is low by international standards and evidence from parathyroid hormone status (which rises with vitamin D deficiency) as well as large population studies of all-cause mortality suggests that a target blood level of at least 50 nmol l−1, as indicated by the American Institute of Medicine and by the European Food Safety Authority, is more appropriate.