Hypocalcemia (Low Calcium Levels) in Cats

What is hypocalcemia?

The term hypocalcemia is used when the level of calcium in the blood is lower than normal (hypo = “below”, -emia = “blood”). Calcium is a mineral found in small quantities throughout the body. It plays an important role in diverse and vital functions, including muscle contraction, transmission of nerve impulses, blood clotting, and bone strength.

How are levels of calcium controlled in a healthy animal?

Calcium levels are controlled by a combination of hormones: parathyroid hormone (PTH), vitamin D (calcitriol), and calcitonin. Parathyroid hormone is produced by a pair of parathyroid glands. 

These two tiny glands are embedded in the thyroid gland, which sits just below the larynx, or "voice box," overlying the windpipe. They produce PTH when blood calcium levels are too low, acting to return calcium levels to normal. 

Vitamin D is absorbed in the diet and helps PTH to maintain calcium levels in the blood. Calcitonin is made in the thyroid gland and acts to decrease calcium blood levels when needed.

What are the clinical signs of hypocalcemia?

Mild decreases in blood calcium concentrations do not usually cause any clinical signs but may be detected on routine blood testing. However, if moderate to severe hypocalcemia develops, then clinical signs may occur. 

Early signs of hypocalcemia include:

• Stiffness, weakness
• Restlessness,  irritability
• Muscle tremors, hypersensitivity (exaggerated responsiveness) to touch and sound

More profound signs include severe generalized muscle twitching, leading to uncontrolled muscle spasms, seizures, and ultimately death.

What causes hypocalcemia?

In cats, hypocalcemia can be caused by multiple conditions, including: 

• Primary hypoparathyroidism (the glands don’t produce enough PTH).
• Decreased vitamin D intake from dietary insufficiency or intestinal malabsorption.
• Chronic renal failure (CRF; also called chronic kidney disease or CKD). However, hypocalcemia caused by CRF is rarely clinically significant (i.e., muscle tremors, twitches, tetany, or convulsions do not develop). Fortunately, most pets with CRF have normal serum calcium concentrations.
• Post-operative complication of thyroid surgery for hyperthyroidism (a common condition in older cats) where the parathyroid gland, located very near the thyroid gland, may be inadvertently damaged or removed.

There are other, less common causes of hypocalcemia, including hypocalcemia associated with lactation in a nursing mother (known as tetany, eclampsia, or milk fever), low blood protein levels (hypoproteinemia), pancreatitis, and certain drugs and toxins including antifreeze.

With hypocalcemia due to chronic renal failure, treatment is directed at lowering the serum phosphate concentrations by restricting phosphorus in the diet and giving medications that prevent dietary phosphorous from being absorbed by the GI tract (phosphate binders).

How is calcium measured?

Two forms of calcium are found in the blood: total calcium and ionized calcium (also called free calcium).

Total calcium: The test for total calcium is typically a preliminary test. However, total calcium can appear falsely decreased due to low levels of albumin (a blood protein that carries calcium around in the blood stream), as well as delayed testing (i.e., sample left standing on the counter for too long before testing). If preliminary testing reveals hypocalcemia, then measuring ionized calcium is often recommended to confirm the finding.

Ionized calcium: Ionized calcium is the definitive test for measuring blood calcium levels, but is more difficult to perform than total calcium, and requires patient preparation and special sample handling. It also tends to be more expensive and often takes longer to get results back from the laboratory.

What further testing is required if my pet has low total calcium on a routine blood test?

If a routine blood test reveals low total calcium, the test is often repeated after a short period of time to confirm the result, especially if albumin levels are normal. The pet should be fasted for 12 hours beforehand but must have access to water. Your veterinarian may recommend repeating other tests as well. If the repeated value for total calcium is still low, then doing an ionized calcium test is recommended since it is the definitive test for measuring calcium levels. In some situations, your veterinarian may recommend the ionized calcium test immediately after finding a low total calcium.

Once hypocalcemia is confirmed, the challenge is to identify the underlying cause. Further testing will be needed to assess the health of the kidneys, the pancreas, the digestive system, and the parathyroid glands, as well as tests to look for evidence of antifreeze poisoning and nutritional problems. Although very rare, hypocalcemia in a nursing cat is easily diagnosed in the period after giving birth, and a single low test is diagnostic.

Why is measuring parathyroid hormone (PTH) important?

Since the parathyroid glands are responsible for controlling calcium levels, measuring parathyroid hormone (PTH) levels is the easiest way to determine if the parathyroid glands are working properly. If a pet has both hypocalcemia and low levels of parathyroid hormone, and there is no evidence of other disease, then primary hypoparathyroidism is the likely diagnosis. 

How is hypocalcemia treated?

Treatment depends on the severity of hypocalcemia and the underlying disease. If hypocalcemia is severe, immediate calcium injections are required to correct the decreased levels. Most cats with severe hypocalcemia must be hospitalized until calcium levels stabilize, as this condition is potentially life threatening.

Long-term treatment of hypocalcemia may include supplementing the diet with calcium salts, and administering vitamin D (tablets, capsules, or liquid), which increases the absorption of calcium from the intestinal tract. Your veterinarian will regularly monitor blood calcium concentrations to ensure successful treatment. The duration of treatment depends on the underlying cause of the condition.