Hypercalcemia: Diagnosis and Treatment Options in the Dog and Cat

Introduction

Calcium is required for a number of intra and extra- cellular functions, as well as for skeletal support. Total calcium (tCa) is the form of calcium most commonly analyzed. Circulating calcium exists in three fractions: ionized, complexed (bound to phosphate, bicarbonate, etc.), and protein-bound (1). In normal dogs, ionized, complexed and protein-bound calcium account for 55%, 10%, and 35% of serum total calcium respectively (2). In cats, values are similar (3). The ionized calcium (iCa) fraction is the biologically active fraction that is regulated and so is the gold standard for evaluation of calcium status. 

Regulation of serum calcium is complex and involves the integrated actions of parathyroid hormone (PTH), vitamin D metabolites, and calcitonin (4). PTH is involved in the fine regulation of blood calcium concentration. When ionized calcium concentration decreases, PTH production is stimulated within the main cells of the parathyroid gland. PTH increases blood calcium concentration by a variety of effects; it also causes phosphaturia, which decreases serum phosphorus. Calcitonin is a minor player in calcium metabolism, serving mainly to limit the degree of hypercalcemia following a calcium-rich meal.

In hypercalcemia, the interaction of calcium and phosphorus is important. Whenever the product of the tCa (mg/dL) multiplied by the phosphorus concentration (mg/dL) exceeds ~70, tissue mineralization is likely. This is critical since the soft tissues that most often mineralize are found in the kidneys, stomach and vascular system.

Key Points

• For accurate detection of hypercalcemia, it is imperative to measure ionized calcium.
• The most common cause of hypercalcemia in the dog is neoplasia.
• An apparently healthy dog with chronic hypercalcemia and hypophosphatemia probably has primary hyperparathyroidism. 
• An apparently healthy cat with chronic hypercalcemia probably has idiopathic hypercalcemia.