Calcium disorders

Parathyroid hormone (PTH) and ionised calcium (iCa) measurements are the mainstay of calcium investigations and their combined measurement should be considered in both hypocalcaemic and hypercalcaemic situations where the cause is not immediately obvious. Total calcium measurement may not provide a true reflection of calcium status in all cases. This is particularly so in animals with compromised renal function. Sample preparation is critical for the accurate analysis of PTH and ionised calcium: separated EDTA plasma shipped frozen for PTH and separated serum for ionised calcium (see sample preparation later in the section. The combination of iCa and PTH measurement usually permits differentiation between the four main categories of calcium regulation disorders:

• Primary hyperparathyroidism (parathyroid dependent hypercalcaemia): functional parathyroid neoplasia
• Parathyroid independent hypercalcaemia: hypercalcaemia of malignancy (for example lymphoma apocrine gland adenocarcinoma), vitamin D toxicosis (calciferol containing rodenticides, calcitriol), Addison’s disease, feline idiopathic hypercalcaemia* and granulomatous disease processes