When it comes to glandular problems in the cat, the thyroid gets all the glory. Hyperthyroidism is the most common feline endocrine disorder, and every cat-centric publication regularly features articles about it. Adjacent to the thyroid gland, however, are four small glands called the parathyroid glands. These little glands are immensely important in finely regulating the blood calcium level.
There are two pairs of parathyroid glands in the cat, the external and the internal parathyroids. The external glands are located external to the capsule that surrounds the nearby thyroid gland, whereas the internal parathyroids are actually embedded within the thyroid gland.
The parathyroid glands produce a hormone called, not surprisingly, parathyroid hormone (PTH). This hormone is the principal hormone involved in the precise, minute-to-minute regulation of the blood calcium concentration. The goal of the body is to maintain the blood calcium level within a narrow range. The parathyroid glands are exquisitely sensitive to changes in the calcium level, especially when the calcium level drops. When this happens, the parathyroids release PTH. This causes several complicated things to occur:
1. PTH causes the bones to release calcium (and phosphorus) into the blood stream.
2. PTH causes the kidney to produce increased amounts of an enzyme that promotes production of a hormone called calcitriol. Calcitriol causes the intestines to absorb more calcium (and phosphorus) from the diet.
3. PTH causes the kidneys to absorb more calcium from the urine, and excrete all that extra phosphorus that came from the bones and the intestinal tract.
The end result is the restoration of a normal calcium level. When the calcium level returns to normal, “mission accomplished” is signaled to the parathyroids, and they reduce the PTH secretion.
Sometimes, however, the parathyroid glands produce too much PTH. This condition is called hyperparathyroidism. Hyperparathyroidism exists in two forms: primary and secondary. Secondary hyperparathyroidism can be further divided into nutritional secondary hyperparathyroidism (pretty rare) and renal secondary hyperparathyroidism (rather common). The focus of this article is primary hyperparathyroidism.
How it happens
Primary hyperparathyroidism occurs as a result of one of the parathyroid glands secreting excessive amounts of PTH on its own. The cause of the excessive secretion is usually due to a benign tumor of one of the glands — called an adenoma — although, in rare instances, the tumor is a malignant carcinoma. Affected cats tend to be older, the age range being somewhere between eight and 15 years of age. Clinical signs tend to be non-specific and include lethargy, poor appetite and vomiting.
Physical examination of the cat tends to be unremarkable, with the only consistent finding to be the detection of the enlarged parathyroid gland in the neck (in approximately 50 percent of the cases). Routine blood tests will show an elevated calcium level.
Because PTH causes the kidneys to excrete phosphorus, some cats with primary hyperparathyroidism will have low serum phosphorus levels. In some cats, the elevated calcium levels will result in the formation of calcium oxalate stones in the bladder. These cats will have signs related to this, such as increased frequency of urination, straining to urinate, urinating in inappropriate places and blood in the urine.
How it’s diagnosed
Definitive diagnosis of primary hyperparathyroidism requires measurement of the serum level of PTH, along with measurement of serum ionized calcium (iCa). Ionized calcium provides a more accurate assessment of the calcium status. If the PTH level and the iCa level are both high, the diagnosis is obvious. If the iCa is high and the PTH is in the upper half of the reference range, the cat probably still has hyperparathyroidism. This is because when the calcium level is high, the proper response of the parathyroid gland is to shut down production of PTH. A PTH level in the upper half of the reference range — in the face of an elevated calcium — is an inappropriate response, suggesting that the parathyroid gland has gone haywire and is secreting the hormone autonomously.
A diagnosis is trickier when the iCa is high and the PTH level is in the lower half of the reference range. Unfortunately, there is no real consensus on what is an appropriately low PTH level in the face of an elevated iCa. Certainly, if the iCa is high and the PTH level is undetectable, then hyperparathyroidism is probably not the cause of the high calcium. But a low normal PTH level in the face of high iCa is a diagnostic dilemma. Ultrasound of the thyroid region of the neck may reveal a single enlarged parathyroid gland, which would be highly supportive of the diagnosis. Not finding an enlarged gland, however, still doesn’t definitely rule it out.
The most commonly recommended treatment of primary hyperparathyroidism is parathyroidectomy — surgical removal of the abnormal gland. Ultrasound of the neck helps identify the exact location of the tumor, i.e. whether it is in one of the external parathyroids or the internal parathyroids. This allows for proper pre-surgical planning. Anesthetic complications related to elevated calcium levels include abnormally slow heart rate, high blood pressure and cardiac arrhythmias, so careful anesthetic planning is an absolute must.
At surgery, one large parathyroid gland is usually found. As a result of that one gland producing such high amounts of PTH, the other three glands have shut off production of PTH completely, causing them to atrophy and making them impossible to identify. A tumor of one of the external parathyroid glands is usually easy to identify (unless it is embedded in fat, making the identification a bit trickier).
Parathyroid tumors involving one of the internal parathyroids are harder for the surgeon to identify. If an internal parathyroid is affected, removal of that entire lobe of the thyroid gland is required. In dogs, a method of treatment called chemical ablation has been described, which involves the injection of ethanol into the abnormal gland. However, studies of the effectiveness of this method of treatment in cats are still lacking.
Post-operative complications occur occasionally, and ironically, the most common is a low calcium level. (However, this is a bigger problem in dogs than in cats.) As mentioned above, excessive production of PTH by a tumor in one parathyroid gland results in atrophy of the remaining parathyroid glands. Removal of the offending gland will cause a rapid drop in blood PTH levels.
It takes a little while before the remaining parathyroid glands “wake up” and start producing PTH again (typically two to three weeks). Until that happens, it is essential that the cat be monitored for clinical signs of hypocalcemia (low calcium). Cats that experience low calcium post-operatively can be treated with a combination of calcium supplements and vitamin D. The supplementation can be tapered over a few weeks as the remaining parathyroid glands begin to function normally. — Arnold Plotnick, DVM, DACVIM