Association of Vitamin D Metabolites with Parathyroid Hormone, Fibroblast Growth Factor‐23, Calcium, and Phosphorus in Dogs with Various Stages of Chronic Kidney Disease

BACKGROUND: Hypovitaminosis D is associated with progression of renal disease, development of renal secondary hyperparathyroidism (RHPT), chronic kidney disease‐mineral bone disorder (CKD‐MBD), and increased mortality in people with CKD. Despite what is known regarding vitamin D dysregulation in humans with CKD, little is known about vitamin D metabolism in dogs with CKD. OBJECTIVES: The purpose of our study was to further elucidate vitamin D status in dogs with different stages of CKD and to relate it to factors that affect the development of CKD‐MBD, including parathyroid hormone (PTH), fibroblast growth factor‐23 (FGF‐23), calcium, and phosphorus concentrations. METHODS: Thirty‐seven dogs with naturally occurring CKD were compared to 10 healthy dogs. Serum 25‐hydroxyvitamin D [25(OH)D], 1,25‐dihydroxyvitamin D [1,25(OH)(2)D], and 24,25‐dihydroxyvitamin D [24,25(OH)(2)D], and PTH and FGF‐23 concentrations were measured. Their association with serum calcium and phosphorus concentrations and IRIS stage was determined. RESULTS: Compared to healthy dogs, all vitamin D metabolite concentrations were significantly lower in dogs with International Renal Interest Society (IRIS) stages 3 and 4 CKD (r [creatinine]: −0.49 to −0.60; P < .05) but not different in dogs with stages 1 and 2 CKD. All vitamin D metabolites were negatively correlated with PTH, FGF‐23, and phosphorus concentrations (r: −0.39 to −0.64; P < .01). CONCLUSIONS AND CLINICAL IMPORTANCE: CKD in dogs is associated with decreases in all vitamin D metabolites evaluated suggesting that multiple mechanisms, in addition to decreased renal mass, affect their metabolism. This information could have prognostic and therapeutic implications.