When to see a vet urgently
Seek prompt veterinary assessment if your cat has any of the following symptoms:
- repeated attempts to urinate with no urine or very little produced;
- marked weakness, falling over or seizures;
- persistent vomiting and failure to eat;
- a sudden deterioration in general condition and an abnormal response to their surroundings.
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Overview
Calcium is an important mineral involved in muscle function, the nervous system and blood clotting. Its level is tightly regulated by hormones, including parathyroid hormone, calcitonin and vitamin D. In idiopathic hypercalcaemia, calcium balance is disrupted without a clear cause and the level remains high for a long time. This leads to kidney damage, urinary stones and problems throughout the body.
Complications are possible, but they do not develop in every cat in the same order or at the same speed. Some cats have no symptoms at diagnosis. The significance is assessed according to the calcium level, whether it remains raised, the symptoms and any kidney changes or urinary stones. The diagnosis alone does not determine an individual cat's prognosis.
Calcium occurs in several forms in the blood. Some is bound to proteins, some to other compounds, and some is free, or ionised. The ionised portion directly affects cell function. A routine total calcium result represents all these portions together and does not always show the level of the active portion.
Causes
Because the diagnosis is made by exclusion, the precise cause is unknown. Possible mechanisms include:
- individual differences in calcium metabolism;
- disturbances in vitamin D metabolism;
- hidden immune or endocrine factors;
- inherited predisposition.
Idiopathic means that adequate investigations have not identified a recognisable cause. It does not mean that every other cause of high calcium is the same disease. Excessive vitamin D intake, parathyroid disease and certain tumours, for example, can raise calcium through different mechanisms. Identifying one of these would change the diagnosis and treatment plan.
Risk factors
Idiopathic hypercalcaemia occurs more commonly in:
- middle-aged and older cats;
- neutered cats;
- cats with chronic digestive problems or reduced activity;
- breeds in which metabolic disorders are more common, such as Siamese cats.
Information on individual predisposing factors in cats is limited. Idiopathic hypercalcaemia can be diagnosed at different ages, and the risk list above does not mean that neutering or low activity, for example, has been shown to cause it. An abnormal result in a young cat or a cat of another breed should not be left uninvestigated on this basis.
Symptoms
Clinical signs depend on how much the calcium level has risen:
- increased thirst and frequent urination;
- reduced appetite and weight loss;
- tiredness and listlessness;
- vomiting and constipation;
- difficulty urinating and pain associated with urinary stones;
- weakness and poor coordination;
- in severe cases, abnormal heart rhythms and seizures.
A mild change may be found during another investigation without any symptoms noticed at home. In another cat, reduced appetite and weight loss may be the most obvious signs. Constipation or urinary stones can also prompt investigations. An absence of symptoms does not confirm that ionised calcium is normal.
High calcium can disrupt kidney function and promote stone formation. Conversely, the different calcium fractions can also change with kidney disease. Their relationship is investigated through tests: simultaneous abnormalities in kidney values and total calcium do not establish which problem came first.
Complications and consequences
Long-standing hypercalcaemia can lead to:
- urinary stones containing calcium;
- chronic kidney disease;
- abnormal heart rhythms and cardiac complications;
- mineralisation of soft tissues;
- chronic weight loss and exhaustion.
Diagnosis at the vet
Diagnosis is based on:
- blood biochemistry, including raised total and ionised calcium;
- urine examination for calcium crystals and urine concentration;
- ultrasound of the kidneys and bladder for stones and mineral deposits;
- ruling out secondary causes, including tumours, hyperparathyroidism, kidney disease and excessive vitamin D intake.
Total and ionised calcium do not both have to be high for an abnormality in active calcium to be significant. Total calcium can also rise without an increase in the ionised fraction. A suspected disorder is therefore confirmed by measuring ionised calcium. A single abnormal result in a cat without symptoms may be checked with another sample.
Sample handling affects the ionised calcium result. Follow the practice's preparation instructions and explain what your cat has eaten and received as medicines or supplements. The vet and laboratory manage the sampling method. A corrected value calculated from total calcium does not replace direct measurement of ionised calcium.
The purpose of hormone tests
Parathyroid hormone, or PTH, is produced by the parathyroid glands and regulates calcium levels. When calcium rises, PTH secretion should normally decrease. Its result is interpreted alongside ionised calcium measured at the same time. Considering a hormone value separately from calcium can lead to a wrong conclusion.
A cat's PTH result may be difficult to interpret even when it falls within the laboratory reference interval. Some cats with idiopathic hypercalcaemia have hormone concentrations within this range. Other findings and possible repeat testing then help distinguish a parathyroid disorder from other causes. A single hormone value within the reference interval therefore does not settle the diagnosis.
PTHrP is a protein similar to parathyroid hormone that is produced by certain tumours. Measuring it can help investigate the cause, but a negative result does not rule out every tumour. Imaging and cell or tissue samples are added if needed. Not all tumours that raise calcium use the same mechanism.
The history includes the entire diet and possible access to harmful substances. Mention vitamin products, mineral supplements and any other products being used. If another disease or exposure explains the finding, treatment targets that cause. An idiopathic diagnosis is made only after these alternatives have been assessed adequately.
Treatment and long-term monitoring
The urgency of treatment depends on both your cat's condition and the severity of the calcium increase. A cat with severe symptoms may need fluid therapy and calcium-lowering treatment in hospital. In a mild, stable situation, dietary assessment and monitoring can proceed as planned. The vet decides whether monitoring alone is sufficient for the individual cat.
Dietary assessment
A dietary change may benefit some cats. Calcium content and its ratio to phosphorus can affect treatment planning. A stated purpose on the packaging, such as supporting the kidneys or urinary tract, does not alone show whether the composition suits a cat with hypercalcaemia. A vet with expertise in nutrition may be involved in planning the diet if needed.
In a small series of cats, raised ionised calcium levels fell after a planned dietary change. Some cats reached a normal level, while others had only a partial response. This finding supports assessing the diet but does not guarantee the same result for every cat. The response is checked with a blood test as well as by observing how well the cat eats.
Calcium and phosphorus intake are assessed as part of the whole diet. Supplements and other foods are counted alongside the main food. A very restricted diet put together without advice may be nutritionally deficient, and the dietary study is not an instruction to remove an essential mineral completely. Changes are planned so that your cat continues to receive the nutrients they need.
Monitoring medication
If diet and monitoring are insufficient, the vet may consider medication that affects calcium metabolism. Bisphosphonates are a group of medicines that reduce calcium release from bone. Their necessity and method of administration are decided individually. Do not change tablet administration instructions at home, because incorrect administration can damage the oesophagus.
Information on the safety of long-term bisphosphonate treatment is limited. Monitoring assesses both the fall in calcium and possible adverse effects, as well as kidney health. The vet adjusts ongoing treatment to the response so that it is not continued unchanged simply because of an earlier diagnosis. Other calcium-lowering medicines may be used in selected situations.
At home, monitor eating, weight, drinking, passing stools and urination. Report new difficulty urinating or repeated vomiting in particular. Follow-up may include kidney values, urine examination and imaging if needed, alongside ionised calcium. The investigations are tailored to whether stones or kidney changes have been identified.
Prevention
There is no specific prevention, but the risk of complications can be reduced through:
- regular blood biochemistry tests for cats over 6–7 years old;
- sufficient water intake and a diet intended to prevent urinary stones;
- careful management of vitamin and mineral supplements, because excessive calcium and vitamin D can worsen the condition;
- regular ultrasound of the bladder and kidneys.
The schedule for imaging and blood tests is agreed individually. There is no single ultrasound interval determined solely by the diagnosis that applies to every cat. Adequate drinking supports urinary tract care but does not replace investigating raised calcium. Discuss supplements and dietary changes with the treating vet.
What it means for owners
Idiopathic hypercalcaemia is a fairly common metabolic disorder in cats and may cause no symptoms in its early stages. Owners should remember that weight loss, thirst and urinary problems are reasons to check calcium levels and kidney function. Early detection slows the development of complications and extends a pet's life.
Note: this material provides general information and does not replace veterinary advice. If symptoms develop, contact a vet immediately.
Frequently asked questions
Can total calcium be normal even if my cat has hypercalcaemia?
Yes. Ionised calcium may be raised even when the total value does not show it. The opposite discrepancy is also possible. The biologically active fraction is therefore tested separately when a calcium balance disorder is suspected.
Does high calcium always mean cancer?
No. Idiopathic hypercalcaemia and changes associated with kidney disease are also possible in cats. Tumours still need to be considered among the causes. A diagnosis is not established from one calcium value or one negative hormone test.
Can diet alone be enough treatment?
In some cats, a planned dietary change can lower ionised calcium sufficiently. Others need additional treatment. Suitable composition and response are assessed with your vet, and the result is confirmed through follow-up samples.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Hypercalcaemia in Dogs and Cats, MSD Veterinary Manual, accessed 2026-11-02.
- Calcium Disorders, Michigan State University Veterinary Diagnostic Laboratory, accessed 2026-11-02.
- Hypercalcaemia, VCA Animal Hospitals, accessed 2026-11-02.
- Hypercalcaemia in Cats - Update on Diagnosis and Treatment, WSAVA 2018 Congress, VIN, accessed 2026-11-02.
- Ionized hypercalcaemia can resolve with nutritional modification in cats with idiopathic hypercalcaemia or chronic kidney disease, Journal of Feline Medicine and Surgery, accessed 2026-11-02.