The parathyroid glands are small endocrine (hormone producing) glands located in the neck behind the thyroid gland. Specialised cells known as 'chief cells' produce parathyroid hormone (PTH) which is responsible for maintaining calcium and to a lesser degree phosphate levels in the blood.
Calcium is the most abundant mineral in the body, it is responsible for many essential functions such as cardiac function, muscle contractions, nerve impulses, blood clotting, stabilises the permeability of the cell membrane to sodium, cell growth, providing strength to the teeth and bones. 99% of calcium is stored in the bones, the remaining 1% is in the blood. The role of the four parathyroid glands is to continually monitor blood calcium levels, if they drop, the glands secrete parathyroid hormone which causes calcium to be reabsorbed from the bones, increases absorption of calcium from the food in the digestive tract and suppresses calcium loss from the urine. If blood calcium levels rise, the parathyroid glands shut off production of parathyroid hormones which in turn decreased bone resorption, decreased intestinal absorption and increased renal excretion (via the urine).
Hypoparathyroidism is a medical condition in which the parathyroid hormone is either greatly decreased or absent. This results in hypocalcemia (low blood calcium levels) and moderate hyperphosphatemia (increased phosphate levels in the blood). It can be divided into several types:
Primary hypoparathyroidism: Most cases of primary hypoparathyroidism in cats are idiopathic (no known cause) atrophy/wasting of the parathyroid gland. Other causes of primary hypoparathyroidism include the destruction of the parathyroid glands by the cat's own immune system (known as immune-mediated parathyroiditis).
Transient hypoparathyroidism: This may develop due to severe magnesium depletion which suppresses secretion and activity of parathyroid hormone. Magnesium is also required for activation of vitamin D in the kidneys. Accidental damage to the parathyroid gland during surgery to remove the thyroid gland may also occur.
Iatrogenic/secondary hypoparathyroidism: This is the most common cause of hypoparathyroidism in cats and is most commonly caused by the accidental damage or removal of the parathyroid glands during surgery to remove the thyroid glands.
As the most common cause of hypoparathyroidism is due to the accidental removal of the parathyroid gland during removal of the thyroid glands, the mean age of cats affected is the same as those cats who have hyperthyroidism, which is typically middle-aged to senior cats. It is seen more in mixed breed cats and males are slightly over-represented.
Symptoms of hypoparathyroidism:
The symptoms of hypoparathyroidism are attributable to hypocalcemia brought on by the reduction/halt to parathyroid hormone levels. Mild hypocalcemia may not produce results at all. It is only when serum levels drop below 6 mg/dl that clinical signs develop.
The most common symptom of hypocalcemia relates to increased neuromuscular activity which results in the following:
- Neuromuscular irritability such as nervousness, whiskers twitching, facial rubbing, tetany (involuntary muscle contractions)
- Facial rubbing
- Stiff legged gait
- Bradycardia (slower than normal heart rate)
- Anorexia (loss of appetite)
- Increased urination
Diagnosis of hypoparathyroidism:
Your veterinarian will perform a complete physical examination and obtain a medical history from you. Recent thyroidectomy would raise his index of suspicion of hypoparathyroidism. Your veterinarian may observe muscle trembling, decreased heart rate and panting. There are a number of other causes of hypocalcemia in cats, so your veterinarian will need to rule these out.
Further tests he may wish to perform include:
- Complete blood count and biochemical profile to rule out other possible medical conditions. This may reveal low blood calcium levels and high blood phosphate levels. The only other condition which causes decreased calcium levels and increased phosphate levels is renal failure, in which case elevated BUN levels should also be present.
- Serum PTH may show low or undetectable levels.
- Exploratory surgery may reveal atrophied or absent parathyroid glands.
How is hypoparathyroidism in cats treated?
Your cat may need to be hospitalised and treated initially until his levels of calcium in the blood have increased.
- Slow intravenous administration of calcium gluconate 10% solution.
- Fluid therapy and nutritional support if your cat is anorexic.
- Vitamin D assists in the absorption of calcium, and therefore may also be given.
- Once the cat has stabilised, oral administration of calcium supplements. Adjustments may be necessary with both vitamin D and calcium in the first few months.
- ECG to check for cardiac abnormalities.
- Regular blood tests to monitor calcium levels to ensure they don't become too low or too high (hypercalcemia). Monthly checks will be necessary for the first six months and then every 2-3 months thereafter.
The long-term prognosis is excellent with proper supplementation and regular monitoring of serum calcium are maintained.
If the parathyroid gland was damaged during surgery to remove the thyroid gland, normal parathyroid gland function may resume in a few months.