PTH and vitamin D deficiency: Chronic vitamin D deficiency is associated with decreased calcium absorption and elevated levels of parathyroid hormone (PTH), which may lead to excessive bone resorption and reduced bone mineral density (BMD).4 The main end points investigated in relation to vitamin D status are therefore serum PTH and BMD.5 The relatively few studies of free S‐25(OH)D in relation to BMD have used the calculated, not directly measured, free S‐25(OH)D and the results are conflicting.6, 7, 8, 9, 10, 11