Rickets in the growing child or adolescent and osteomalacia in the adult develop in a variety of clinical situations and have in common an absence or delay in the mineralization of growth cartilage and in newly formed bone collagen. Classically, deficiency of vitamin D, which is essential for the absorption of dietary calcium, has been the major cause. However, rickets is also seen as a result of hereditary defects in critical vitamin D signaling molecules. Disturbances of phosphate metabolism can also lead to signs of rickets and osteomalacia, notably X-linked hypophosphatemic rickets, and oncogenic osteomalacia. Extrarenal synthesis of 1,25-dihydroxyvitamin D, such as that associated with granulomatous disease, can also lead to disturbances in calcium metabolism, with associated skeletal and nonskeletal changes.
A three-year-old Border collie was presented with a two-week history of lethargy, stiff gait, polydipsia and polyuria. Biochemical analysis revealed hypercalcaemia. Serum concentrations of 25-hydroxyvitamin D (25[OH]D) and 1,25-dihydroxyvitamin D (1,25[OH]2D) were markedly elevated and parathyroid hormone was undetectable. Subsequent analysis of the dog's diet revealed that the food contained excessive amounts of vitamin D. The hypercalcaemia resolved following treatment with bisphosphonates and dietary change. Hypervitaminosis D was diagnosed in a second unrelated dog, which had been fed the same brand of dog food as case 1. The dog was also hypercalcaemic and had markedly elevated serum concentrations of 25(OH)D and 1,25(OH)2D. Hypervitaminosis D in dogs has been reported to occur secondarily to ingestion of either rodenticides containing cholecalciferol or antipsoriatic ointments that contain vitamin D analogues. Hypervitaminosis D has also been reported following the treatment of hypoparathyroidism. To the authors' knowledge, this is the first report of hypervitaminosis D in dogs following the accidental over supplementation of a commercial diet with vitamin D. While the benefits of adequate dietary vitamin D are well established in dogs, the potential deleterious effects of over supplementation of vitamin D should also be acknowledged.
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