Canine hyperparathyroidism is one of several causes of hypercalcemia in dogs.
Hypercalcemia, or elevated calcium level in the bloodstream, is defined as any laboratory reading above 11.4 mg/dl.
In dogs, as in people, the four parathyroid glands are located nestled into the thyroid glands, thus their name. Para is a Latin prefix meaning “beside.” Both are located next to the trachea (windpipe) in the throat. Parathyroid glands should maintain calcium levels between 8.9 and 11.4 mg/dl. Alternatively, when measuring ionized calcium, the active form in the bloodstream, the normal range is 1.24-1.43umoles (micromoles-ionized calcium is measured in different units from total calcium). For a discussion of low- or non-functioning parathyroid glands creating a condition called hypoparathyroidism, click here.
As the calcium level rises, phosphorous in the blood is reduced proportionately, further compounding electrolyte imbalance.
Primary hyperparathyroidism occurs when an adenoma, a benign (non-cancerous) but functional growth in the parathyroid glands begins producing excess amounts of parathormone, or parathyroid hormone (PTH). A small percentage of primary hyperparathyroidism patients have a cancerous growth in their parathyroid glands.
Hypercalcemia in dogs is more likely to occur in neoplasia (cancer) located in non-parathyroid sites in the body.
A test to determine whether the parathyroid glands are the cause of hypercalcemia combines detection of the amount of PTH in the blood and a measurement of ionized calcium. Calcium in its ionized form is the way the body uses it.
To diagnose primary hyperparathyroidism, PTH level is expected to be normal to high. Ionized calcium will also be elevated. In the normal patient, high serum calcium would provide feedback to the parathyroid glands to let them know too much calcium was circulating, and the glands would reduce their production of PTH, and calcium level would drop back to normal. The adenoma in the parathyroid gland, however, does not respond to feedback, and continues pumping out more and more PTH, which causes the body to mobilize even more calcium.
The treatment for primary hyperparathyroidism is to surgically remove the abnormal parathyroid gland. Masses may be detected by ultrasound, magnetic resonance imaging (MRI) or a combination of the two techniques. In most cases, the normal parathyroid glands have atrophied physically and functionally. Such patients may initially be treated as hypoparathyroid, as the body’s usual means of balancing calcium level needs time to return to proper function. Calcium level in the blood must be closely monitored. Most canine patients return to normal calcium regulation within a few weeks.
Primary hyperparathyroidism is most likely to occur in older dogs of either gender. No breed predisposition is known, although the Keeshond does appear in reported cases more commonly than other breeds.
Patients may present with loss of appetite and excessive thirst and urination caused by the effects of high calcium concentration on the kidneys. Urinary tract infection and stone formation in the urinary tract are not uncommon.
See you next week, Dr. Randolph.
