Calcium, Phosphorus, PTH

Quick Answer: Phosphorus and calcium are meant to stay in balance, but damaged kidneys struggle to excrete excess phosphorus, which throws that balance off. Left unaddressed, this can eventually push the parathyroid glands into overdrive, producing a condition called secondary hyperparathyroidism, and in its more advanced form, CKD-mineral and bone disorder (CKD-MBD). Catching the imbalance early, primarily through monitoring phosphorus, calcium, and where possible PTH, is one of the most impactful things you can do for a CKD cat.

Phosphorus and calcium are both minerals stored primarily in bone, and the body works hard to keep them in a fairly fixed ratio to one another. Healthy kidneys excrete excess phosphorus as needed to maintain that balance. As kidney function declines, that excretion becomes harder, phosphorus starts to accumulate, and calcium levels often shift in response, sometimes up and sometimes down depending on the individual cat and how far the disease has progressed.

This page focuses on what the test results mean and why they matter diagnostically. For the practical side of managing phosphorus day to day, including diet and phosphorus binders, see our phosphorus page.

Virtually every CKD cat eventually develops elevated phosphorus (hyperphosphatemia), and it is one of the more consequential imbalances to manage, since persistently high phosphorus can accelerate CKD progression and make a cat feel noticeably worse, with symptoms like nausea and reduced appetite.

Being within a lab’s normal range is not the same as being at a good level for a CKD cat. As a general guide, a phosphorus level above 6 mg/dl (1.9 mmol/L international) calls for action, and the goal for most CKD cats is to bring phosphorus below 4.6 mg/dl (1.5 mmol/L international), though cats in more advanced stages may not be able to reach that target and the more realistic goal becomes simply staying under 6 mg/dl.

Low phosphorus (hypophosphatemia) is much less common in CKD cats and usually happens when phosphorus binders have been used too aggressively. It is not usually dangerous unless it drops quite low, but persistent low readings are still worth mentioning to your vet.

Calcium exists in the blood in three forms, bound to proteins, complexed with other substances, and ionized, or free. The ionized fraction is the biologically active portion and is what actually matters most for a CKD cat’s health, but it is measured far less often than total calcium because it requires a specialized test that not every laboratory can run.

This distinction matters because total calcium and ionized calcium do not always move together. A cat can have a normal total calcium reading while actually having elevated or reduced ionized calcium underneath it, meaning total calcium alone can miss real problems. If your cat’s total calcium is persistently unusual, or if secondary hyperparathyroidism is suspected, it is worth asking specifically about ionized calcium testing.

High calcium (hypercalcemia) is more commonly seen than low calcium in CKD cats. A total calcium above roughly 11 mg/dl (2.75 mmol/L) is generally considered elevated, though symptoms typically do not appear until levels climb well beyond that. When no clear cause can be found, this is referred to as idiopathic hypercalcemia, which turns out to be the most common form seen in cats generally. The usual first step when hypercalcemia shows up is simply to repeat the test, ideally after a period of fasting, before pursuing anything further.

Low calcium (hypocalcemia) happens because damaged kidneys are less able to activate vitamin D into its usable form, which reduces how much calcium the body absorbs and releases from bone stores. It is generally considered present when total calcium falls below about 9 mg/dl, and it often occurs alongside elevated phosphorus, since high phosphorus itself can further suppress the vitamin D activation process.

Beyond looking at phosphorus and calcium individually, it is also useful to look at the two multiplied together, since a high combined product increases the risk of a serious complication called calcification, where soft tissue throughout the body starts to harden.

As a rough guide, a product above 60 in US values (or 5 in international values) using total calcium suggests some risk of calcification, and the risk is generally considered more serious when the elevated product is driven mainly by high phosphorus rather than by high calcium. This is one more reason phosphorus control tends to be the priority even when calcium is also somewhat elevated.

The parathyroid glands regulate phosphorus and calcium through two hormones, parathyroid hormone (PTH) and calcitriol, the active form of vitamin D. In a healthy cat, if phosphorus rises or calcium falls, the parathyroid glands release more PTH, which pulls calcium out of bone and pushes the kidneys to excrete more phosphorus, restoring balance and then stopping.

In a CKD cat, this feedback loop starts to break down. The kidneys can no longer produce enough calcitriol to properly signal the parathyroid glands to stand down, so PTH keeps climbing even as phosphorus continues to build. Research suggests this process can actually begin before phosphorus itself becomes abnormal, meaning a cat can be developing secondary hyperparathyroidism while phosphorus still looks normal on a standard panel. This is part of why relying on phosphorus alone can miss what is happening earlier in the disease process.

Left unaddressed, secondary hyperparathyroidism can contribute to a range of problems beyond the mineral imbalance itself, including reduced appetite, weakened immunity, and interference with the hormone that stimulates red blood cell production, making it a contributing factor in some cases of CKD-related anemia.

When secondary hyperparathyroidism progresses far enough, it can develop into CKD-mineral and bone disorder (CKD-MBD), a broader syndrome involving mineral imbalance, bone changes, and calcification of soft tissue. Reported sites of calcification in CKD cats include the aorta, stomach wall, kidneys, and paws, and the condition can be genuinely painful as well as damaging to organ function.

A related but rarer complication is a softening of the jawbone, sometimes called renal osteodystrophy, though this shows up far more often in dogs than in cats.

The takeaway for most owners is not that every cat with elevated phosphorus is heading toward CKD-MBD, since with good phosphorus management most never get there, but that early and consistent monitoring of phosphorus (and PTH where available) gives you the best chance of staying ahead of it.

Most general veterinary laboratories cannot run PTH or ionized calcium testing, so these usually need to go to a specialist laboratory. In the USA, Michigan State University’s veterinary diagnostic laboratory is a commonly used option for both tests together, and your cat typically needs to fast for around eight hours beforehand to avoid interference from fat in the blood sample. In the UK, some specialist laboratories offer similar testing, though availability is more limited than in the USA.

Because these tests require careful sample handling and are not run daily at every lab, turnaround time is usually a few days rather than the same-day results you might get from an in-house chemistry panel.

My cat’s total calcium is normal. Does that mean I don’t need to worry about ionized calcium? Not necessarily. Total and ionized calcium do not always track together, and a normal total calcium can sometimes sit alongside an abnormal ionized calcium underneath it. If your vet has any reason to suspect a calcium problem, ionized calcium is worth asking about specifically.

Is secondary hyperparathyroidism reversible? The underlying kidney damage driving it is not, but the hormonal imbalance itself can often be brought back under better control with consistent phosphorus management, which is why catching it early matters so much.

My cat’s phosphorus is in the “normal” range on the lab report. Is that good enough? Not always. Lab reference ranges are built from a general healthy population, not specifically for CKD cats, and the target for a CKD cat is generally lower than the top of that range. It is worth asking your vet what target makes sense for your cat’s specific stage.

Can diet alone fix a high phosphorus times calcium product? Often it helps significantly, particularly a therapeutic kidney diet, but how much it can achieve depends on how advanced the CKD is and what is driving the elevated product. This is best worked through with your vet, including whether phosphorus binders are also needed.

• A review of the role of fibroblast growth factor 23 in phosphate homeostasis and the pathophysiology of mineral bone disorder associated with chronic kidney disease (2023) Elliott J, Geddes R & Jepson R International Renal Interest Society
• Hypercalcemia in chronic kidney disease (2024) Van den Broek H International Renal Interest Society
• Chronic kidney disease in cats and the risk of total hypercalcaemia (2017) van den Broek DHN, Chang Y-M, Elliott J & Jepson RE Journal of Veterinary Internal Medicine 31(2) pp465-475
• Prediction of serum ionized calcium concentration by serum total calcium measurement in cats (2010) Schenck PA & Chew D Canadian Journal of Veterinary Research 74(3) pp209-213
• Parathyroid hormone concentration in geriatric cats with various degrees of renal function (2012) Finch NC, Syme HM & Elliott J Journal of the American Veterinary Medical Association 241(10) pp1326-1335
• Feline chronic renal failure: calcium homeostasis in 80 cases diagnosed between 1992 and 1995 (1998) Barber PJ, Elliott J Journal of Small Animal Practice 39 pp108-116
• Ionized hypercalcemia in cats with azotemic chronic kidney disease (2012-2018) van den Broek DHN, Geddes RF, Lötter NS, Chang Y-M, Elliott J & Jepson RE Journal of Veterinary Internal Medicine 36(4) pp1312-1321