PTH and vitamin D deficiency: Thus, the primary aetiology of vitamin D deficiency in CD is its malabsorption, with an intestinal mucosal lesion being, however, also a decisive factor for hypovitaminosis D. Calcium absorption in the intestine, reabsorption from the kidney, and exchange from bones is tightly controlled through the coordinated actions of calciotropic hormones, mainly parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D (1,25(OH)2D3).