Vitamin D (25OHD) and parathyroid hormone (PTH) are important in both bone health and disease. In the past decade, there has been an increasing appreciation of the complexity and importance of their regulation and actions. Widespread prevalence of Hypovitaminosis D has been identified worldwide. Surprisingly it is more common in people living in tropical zones than inhabitants of the countries where sunshine exposure is considered insufficient. Countries at low latitude have also shown a high prevalence of vitamin D deficiency, ranging from 50 to 97%. The suggested mechanism is customary clothing in countries like Middle East leading to a high prevalence of vitamin D deficiency. Similarly, a high prevalence of vitamin D deficiency is also reported from Pakistan. Levels of 1,25-dihydroxyvitamin D (1,25(OH)2D), which is the biologically active form of vitamin D, are tightly regulated by parathyroid hormone (PTH), phosphate and calcium. Vitamin D deficiency causes a compensatory increase in PTH, in an effort to maintain calcium (secondary hyperparathyroidism). However, not all patients with hypovitaminosis D will necessarily show hyperparathyroidism. The term ‘vitamin D insufficiency’ defines a level at which the decreased vitamin D causes an increase in PTH. It has been suggested that the lowest vitamin D level that does not produce a rise in PTH should be selected as optimal. Magnesium deficiency may cause a blunted response to PTH. Assumptions about vitamin D status should not be made based on PTH and calcium values, and 25(OH) vitamin D measurements should be requested when vitamin D deficiency is clinically suspected, irrespective of biochemical results. This depends on cost-benefit ratios that various physicians ascribe to testing for deficiency versus treating for suspected deficiency. Vitamin D deficiency can have several causes, and the pattern of laboratory values with respect to 25-hydroxyvitamin D and PTH and response to vitamin D supplementation may help lead to identification of an underlying pathology.
