Dogs and cats each have four parathyroid glands, two on each side of the thyroid. Their job is to control calcium levels in the animal’s body. The high level (hyperparathyroidism) causes an increase in calcium levels; the low level (hypoparathyroidism) causes low calcium levels—sometimes dangerously low.
Unusual in dogs, parathyroid disease is downright rare in cats. Strangely, in one study, five of seven cats with hyperparathyroidism were Siamese, and four out of five cats with hypoparathyroidism were males. Older pets (9 to 12 years old) seem to be more likely to be affected by high parathyroidism (PTH), but hypoparathyroidism is more common in middle-age pets.
There are two kinds of high PTH: Primary high PTH is largely the result of tumors on the parathyroid gland. As the elevated level of calcium goes unregulated and unchecked, the rest of the body gets the brunt of this growing snowball. This condition also can be caused by parathyroid tissue being located in an abnormal spot in the body, secreting the hormone on its own, away from the neck. This is called an ectopic location, and like an ectopic pregnancy, stuff isn’t where it’s supposed to be. It pays to know what’s causing the high calcium levels. The causes are widespread, so have your vet sort out which cause is parathyroid related and which isn’t.
Secondary types of hyperparathyroidism are much more complex in nature. I’ll do a brief breakdown of the two usual suspects: Kidney-induced secondary high PTH is one and nutritional secondary is the other. In nutritional secondary hyperparathyroidism, the dog is usually younger and has a horrible diet. When a pet is malnourished, they have low calcium, high phosphorous levels and low vitamin D. But because of the great advances in pet nutrition, we just don’t see this type much anymore.
The nutshell on the kidney version is more of the chemical kind: The animal goes into renal failure, which causes all kinds of problems. But for now, I’ll stick to the ones that relate to this topic. The kidney failure wreaks havoc on Vitamin D, as well as the ability to get rid of phosphorous. Both issues can cause calcium levels to drop, which in turn, causes the PTH to go into overdrive to make up for it. Low vitamin D means less calcium absorbed and lack of stimulation of the PTH. Low calcium and no feedback from vitamin D causes the PTH to go crazy with its production.
On the opposite side, hypoparathyroidism almost always occurs following treatment for primary hyperparathyroidism, the surgical removal of the offending parathyroid. Often it is temporary and the other glands kick in post-operatively. You can have autoimmune destruction of the parathyroid gland, we think, but it doesn’t seem common.
Think of it this way: Hyperpara thyroidism makes high calcium levels. Symptoms tend to be vague with the traditional triad of lethargy, depression and inappetence. Drinking more and urinating more can be signs, too, but they also are signs of lots of other things. Hypopara thyroidism causes low calcium and high phosphorous levels. This can seriously mess with the heart and nervous system, and can cause fast heart rates, even arrhythmias. Your pet also will drink and urinate more. The nervous system signs include twitching and muscle spasms, which often impair walking and disrupt other movements.
Diagnostically, blood work is very helpful in delineating the calcium levels, as well as other minerals and electrolytes. This gives your vet clues as to which causes of high or low calcium levels may be parathyroid gland-related and appropriately proceed with more helpful and logical diagnostics, like ultrasounds or x-rays.
Once more, we have high levels of one hormone creating one set of problems, whether primarily caused, or as a result of something else. Keep a watch for problems in your pets, but, for a change, we can be grateful these highs and lows are not very common. LN