Blood calcium does not fluctuate in the growing cat. There are several postulated mechanisms for the impairment in renal concentrating ability. Renal injury and azotemia may develop as a result of direct toxicity to renal tubular epithelium and hypercalcemic induced vasoconstriction of renal vessels leading to ischemia. It is usually not difficult to identify the cause of elevated serum calcium. Pseudohyperparathyroidism due to lymphoma and perirectal tumors are the most common causes of hypercalcemia.
Primary hyperparathyroidism occurs infrequently.
The history may provide information about possible rodenticide exposure. A physical exam will often disclose enlarged lymph nodes in patients with lymphoma although some forms of lymphoma such as the intestinal form may not have abnormalities detectable by examination. Rectal exam will often detect perirectal tumors, although some can be quite small so careful palpation is necessary. The magnitude of elevation of hypercalcemia, phosphous level and PTH level can all be used to differentiate between the causes of hypercalcemia. Additionally PTH can be measured and is increased in patients with primary hyperparathyroidism and renal failure.