Parathyroid hormone works along with the hormone calcitonin and Vitamin D to ensure that the proper levels of calcium stay consistent despite the ingestion of foods and the expelling of waste. If levels of PTH are abnormally low, the condition is known as “hypoparathyroidism”. This problem is rare in cats, but can be found in middle aged to senior felines, with slightly more males being diagnosed than females. Hypoparathyroidism leads to hypocalcemia (low calcium levels) which affects the entire body in a negative way. 

The parathyroid hormone (often shortened to PTH) is secreted by small endocrine glands found behind the thyroid glands in the neck. These glands are called “parathyroid glands”. The main role of PTH is to control calcium levels in the plasma of the body. It also regulates phosphate levels in relation to the amount of calcium in the body. While 99 percent of the body's calcium is found in the bones, the one percent that exists in the plasma is responsible for muscle movement, blood clotting, enzyme production, nerve activity, and hormone release. Thus, when something negatively impacts parathyroid secretion, it in turn depletes the body's calcium levels. 

Often, the largest effect of lowered PTH levels will be seen on the central nervous system of the cat. If levels are only mildly lower than average, no obvious symptoms may exist. Signs to watch for include:

While there is often an underlying cause of lowered PTH levels, some cases may be idiopathic (having no known cause). The issue can have a rapid and spontaneous occurrence. All known causes are listed below.

Your cat's full medical history will be needed to diagnose the condition. Any recent surgeries will need to be disclosed, as this is the most common cause of lowered PTH levels in cats. Your veterinarian will perform a complete physical examination of the cat to evaluate all symptoms that are showing. Full blood work will need to be run to assess all mineral and hormone levels in the bloodstream. A low level of calcium paired with a high amount of phosphate is generally a good indication that parathyroid hormone levels are low. 

Urinalysis can also be helpful in determining how the kidneys are functioning, as failing kidneys many not properly reabsorb calcium from the bloodstream. Pancreatitis may need to be tested for. A removal of part of one parathyroid gland for histopathological examination can be helpful in determining how the glands are working. Any atrophy or destruction of the glands will be noted. The actual level of PTH in the blood may be counted. Other causes of hypocalcemia will have to be differentiated from.

To relieve all negative symptoms that have developed in the cat, proper levels of calcium will have to be restored to the body. Hospitalization is often required for this process.

Supplementation 

Administering both calcium and Vitamin D intravenously will bring a severely depleted cat back to health. Oral administration may suffice in milder instances.

Intravenous Fluids 

IV administration of calcium can lead to hypercalcemia development in the cat. If this happens, the calcium should be stopped for a period of time and replaced with saline and furosemide until heart levels recover. An electrocardiogram may need to be run throughout the IV treatment to ensure the heart rate stays stable. 

If the cause of the decrease in PTH was not able to be diagnosed, the cat may need lifelong supplementation of Vitamin D and calcium to ensure levels are kept high enough in the body. Regular checkup appointments will be needed to test the blood for levels of calcium and to adjust the dose of supplements being given to the cat. This may need to be done every month for the first six months after initial treatment and then every two or three months thereafter.

If the cause of the lowered parathyroid hormone was accidental damage in a surgery, often proper parathyroid gland function will recover at some point in the following months. If the condition is detected early, a good prognosis generally follows. As long as supplements are administered properly and blood work is monitored on a regular basis, the cat can live a long and normal life.