Calcium

A persistently high calcium (hypercalcaemia) is one of the more important abnormalities to pursue, because its causes include serious conditions. A widely used memory aid for the differentials in dogs and cats runs through cancer (some tumours produce a hormone-like substance that raises calcium — a leading cause), kidney disease, Addison's disease (hypoadrenocorticism), overactive parathyroid glands, and vitamin D excess (from certain rodenticides, plants or supplements). A confirmed, persistent hypercalcaemia therefore usually triggers a structured search — often including ionised calcium (the physiologically active fraction, which confirms whether the total is truly raised) and parathyroid hormone — to find the cause.

High calcium can also directly harm the kidneys and cause increased thirst and urination, so it is not only a clue but sometimes a problem in its own right.

A low calcium (hypocalcaemia) matters because, when marked, it disrupts nerve and muscle function — causing tremors, twitching, or in severe cases seizures. Causes include underactive parathyroid glands, kidney disease, certain gut and pancreatic conditions, and, classically, eclampsia in a nursing mother whose milk drains calcium faster than she can replace it. A low total calcium is often driven simply by a low albumin (much calcium is carried on albumin), which is why ionised calcium is measured to tell an apparent low from a real one.

Calcium is closely tied to phosphorus and to kidney health — the two minerals are regulated together, and their balance is disturbed in chronic kidney disease. In a kidney patient, calcium is watched alongside phosphorus as part of managing mineral and bone disturbance. Where a hormonal cause is found and treated, tracking calcium over time confirms the treatment is holding the level where it should be.

Calcium is interpreted the same way in both species, with the differential lists broadly shared. The cancer association (particularly certain lymphomas and anal-sac tumours in dogs) makes a persistent hypercalcaemia a finding neither species can ignore. In both, the crucial refinements are the same: confirm with ionised calcium, correct for albumin, and pursue a persistent, unexplained value rather than acting on a single borderline one.

Total calcium is measured on a routine chemistry panel, in mg/dL (US) or mmol/L (UK/SI). Because much of it is bound to albumin, and because only the ionised (free) fraction is physiologically active, an abnormal total is often followed up with an ionised calcium measurement to confirm it is real. Careful sample handling matters, as some collection factors can alter the reading. Read the value against your own report's range.

A single borderline calcium is often simply rechecked. A confirmed, persistent abnormality — especially a high value — warrants a focused work-up with your vet: ionised calcium to confirm it, then a search through the hormonal, kidney and cancer causes as the picture directs. Because calcium sits under such tight control, a true abnormality is a message worth following to its source.