Calcium Imbalances

Phosphorus gets most of the attention in CKD management, but calcium and phosphorus work as a pair in the body, and problems with one often drag the other along with it. Left unaddressed, an imbalance between the two can lead to a condition called secondary hyperparathyroidism, which is common in CKD cats and worth understanding even if your cat’s bloodwork looks fine right now.

This post covers high calcium, low calcium, and what secondary hyperparathyroidism actually is, along with the treatments used for each. If you haven’t read our phosphorus binders post yet, it’s worth doing so first, since phosphorus control is the foundation everything here is built on.

The body works hard to keep calcium and phosphorus in balance. When phosphorus rises (as it does in most CKD cats) or when calcium falls, the parathyroid glands respond by releasing more parathyroid hormone (PTH). PTH pulls calcium out of the bones and tells the kidneys to excrete more phosphorus, restoring balance. It also stimulates the kidneys to produce a hormone called calcitriol, which further helps raise calcium.

In a healthy cat, this feedback loop works and then shuts back off once calcium is restored. In a CKD cat, damaged kidneys can’t produce enough calcitriol, and rising phosphorus keeps blocking the loop from resolving. PTH stays elevated indefinitely, and over time this persistent overproduction is what’s called secondary hyperparathyroidism.

High calcium is fairly common in CKD cats, though it doesn’t always need active treatment. It becomes a concern when:

• Ionised calcium is also elevated, or

• Total calcium multiplied by phosphorus exceeds roughly 60 to 70 (US units) or 5 (international units)

Either of these raises the risk of soft tissue calcification, which is worth taking seriously.

Treatment Options for High Calcium

Subcutaneous fluids. Most CKD cats with creatinine over 3.5 mg/dl (300 µmol/L) are already on sub-Qs for hydration, and this alone can help lower calcium somewhat. If calcium is a specific concern, your vet may recommend saline solution over lactated Ringer’s, since saline contains no added calcium and helps promote calcium excretion in the urine.

Phosphorus control. Since the two minerals are connected, bringing phosphorus down can help bring calcium down too. This is one more reason phosphorus management matters even in a cat who isn’t currently phosphorus-elevated. Note that calcium-based phosphorus binders should never be used in a cat who already has high calcium.

Dietary review. Not all therapeutic kidney diets have the same calcium to phosphorus ratio, and switching foods or adjusting how much of a therapeutic diet is fed can sometimes help. This is a conversation for your vet, since a full switch away from a kidney diet isn’t right for every cat.

Increased fibre. Extra fibre in the diet can bind calcium in the gut and reduce how much gets absorbed. Chia seeds (around 2g per day, mixed with water and added to food) have shown some benefit in a small case series, though the cats involved only had mild elevations.

Corticosteroids. Prednisolone can lower calcium in some cats, with roughly half of cats with idiopathic hypercalcaemia becoming normocalcaemic on a course of 5 to 10mg per day. The tradeoff is the risk of steroid side effects, including an increased chance of developing diabetes with longer courses.

Bisphosphonates. If calcium remains stubbornly high, drugs like alendronate (given orally, once weekly) or pamidronate (given intravenously) work by encouraging bone to reabsorb calcium from the blood. Alendronate has shown good tolerance and effectiveness in cats with idiopathic hypercalcaemia, though there’s some caution about using bisphosphonates in patients with more severe kidney impairment.

Ruling out cancer. High calcium is occasionally caused by cancer, so if the cause isn’t obvious or treatments aren’t working, it’s worth asking your vet to investigate further.

Low calcium is less common in CKD cats than high calcium, but it does happen. Treatment is usually a calcium supplement such as calcium carbonate, with regular monitoring since overcorrecting into hypercalcaemia is possible. Calcitriol, discussed below, is sometimes used for low calcium as well, typically at very small doses given twice daily.

As explained above, secondary hyperparathyroidism develops when persistently high phosphorus and inadequate calcitriol production keep PTH elevated long term. Left unmanaged, it can contribute to bone changes and worsening mineral imbalances.

Phosphorus Control Is the First Line of Defense

The strongest evidence for preventing or reversing secondary hyperparathyroidism points back to phosphorus control. Feeding a reduced-phosphorus diet, adding binders where needed, has been shown to prevent the rise in PTH that occurs when phosphorus goes unchecked, and cats managed this way in one long-term study lived more than twice as long as those who weren’t. For most cats, disciplined phosphorus management is enough to avoid secondary hyperparathyroidism altogether, or at least significantly delay it.

Magnesium

If bloodwork shows low magnesium alongside secondary hyperparathyroidism, a magnesium supplement may help. Cats with low magnesium tend to have shorter survival times, and a magnesium-enriched phosphate-restricted diet has shown promise in stabilising a related biomarker (FGF23) and preventing hypercalcaemia in a recent controlled trial.

Calcitriol

Calcitriol is the active hormone form of vitamin D, and it’s the most debated treatment on this page. The theory is straightforward: since damaged kidneys can’t make enough calcitriol on their own, supplementing it should help control PTH the way it would in a healthy cat.

In practice, the evidence is mixed. Some research groups have found it effective at controlling PTH, while controlled studies using the same dosing protocols have failed to show a significant difference. As of 2023, IRIS actually removed its previous mention of calcitriol as a treatment for Stage 3 and 4 CKD in cats, citing a lack of evidence for benefit.

If you and your vet do decide to try calcitriol, there are some important prerequisites. Phosphorus needs to be below 6 mg/dl (1.9 mmol/L) before starting, since calcitriol won’t work well otherwise and may worsen phosphorus control. Calcium times phosphorus should also be below the 60-70 (US) or 5 (international) threshold mentioned above, since calcitriol can raise calcium and adding that risk on top of an already elevated calcium-phosphorus product increases the danger of soft tissue mineralisation.

Calcitriol requires compounding into cat-sized doses, since the amounts needed are measured in nanograms, and it’s typically given either daily or on an intermittent schedule (every 3.5 days) once a maintenance routine is established. If you go this route, regular monitoring of PTH, calcium, and phosphorus is essential, both to check whether it’s working and to catch rising calcium early.

Our honest take, matching what a lot of experienced CKD caregivers have concluded: calcitriol isn’t an essential treatment, and for most cats, disciplined phosphorus control alone will do more for secondary hyperparathyroidism than adding calcitriol on top. It’s worth discussing with your vet, but don’t feel you’re failing your cat by skipping it, especially if it’s hard to source or your cat’s phosphorus isn’t yet well controlled.

Calcimimetics

Calcimimetics like cinacalcet are a newer, more experimental option, used in human dialysis patients to manage secondary hyperparathyroidism and occasionally tried in cats when other approaches haven’t worked. Cinacalcet lowers calcium, so it isn’t appropriate for cats who already have low calcium, and it’s expensive with limited feline-specific experience to draw on. This is very much a last-resort conversation with your vet rather than a first-line option.

If your cat is being actively managed for a calcium or PTH issue, expect more frequent bloodwork than a cat who’s stable on phosphorus control alone. Ionised calcium in particular should be checked periodically if your cat is on calcitriol or a bisphosphonate, since these treatments can push levels in either direction depending on dose and response.

Does every CKD cat develop secondary hyperparathyroidism? No. Cats whose phosphorus stays well controlled through diet and binders may never develop it, or may develop it much later in the disease. It’s a consequence of prolonged, poorly controlled phosphorus more than an inevitable part of CKD itself.

My cat’s calcium is slightly high but otherwise seems fine. Do I need to treat it? Not necessarily. Mild elevations that don’t meet the thresholds described above (elevated ionised calcium, or a calcium times phosphorus product above the danger range) often don’t require specific treatment beyond the phosphorus control you should already be doing.

Should I ask my vet about calcitriol? It’s a reasonable question to raise, but go in informed that the evidence for cats is genuinely mixed, and that phosphorus control should come first regardless. IRIS itself pulled back its recommendation for calcitriol in later-stage CKD cats in 2023 due to lack of evidence.

Is secondary hyperparathyroidism reversible? It can be prevented or improved with good phosphorus control, especially if caught earlier rather than later. How much it can be reversed once established varies by cat and depends on how advanced the underlying CKD is.

What’s the difference between calcitriol and a calcium supplement? A calcium supplement adds calcium directly. Calcitriol is a hormone that helps the body absorb and regulate calcium, and it’s specifically aimed at controlling PTH, not at raising calcium on its own, though it can have that side effect.

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