Hypocalcemia (Low Calcium Levels) in Dogs

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 growth.

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 – another gland in the neck – and acts to decrease blood calcium levels when needed.

Why is having low calcium bad for my pet?

Low calcium levels are associated with several serious disorders including antifreeze poisoning, inflammation of the pancreas, kidney failure, and parathyroid gland failure. In nursing female dogs, heavy milk production can lead to hypocalcemia (termed tetany, milk fever, or eclampsia) and may result in seizures. 

What are the clinical signs of hypocalcemia?

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

Pets with abnormally low calcium levels often show signs of muscle twitching, loss of appetite, weakness, and listlessness. In severe cases, pets may have convulsions or seizures.

What causes hypocalcemia?

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

• 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.
• Primary hypoparathyroidism (the glands don’t produce enough PTH).
• Decreased vitamin D intake from dietary insufficiency or intestinal malabsorption.

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.

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 simple, rapid, and relatively inexpensive. It is typically used as a preliminary test to measure calcium levels. 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: The definitive test for measuring blood calcium levels, ionized calcium is an excellent test, 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 at the same time. 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 advise that your pet have the ionized calcium test 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. Hypocalcemia in a nursing female dog is easily diagnosed in the period after whelping, 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 of determining 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. This is usually due to a poorly functioning parathyroid gland.

How is hypocalcemia treated?

Treatment depends on the severity of the hypocalcemia and the underlying disease. For example, primary hypoparathyroidism can be managed with a medication called calcitriol (brand names: Rocaltrol®, Calcijex®) plus nutritional supplements containing vitamin D and calcium. Nursing dogs experiencing hypocalcemia should be treated on an emergency basis with calcium given intravenously. Hypocalcemia that develops secondary to other conditions usually resolves or improves once the underlying problem is treated.