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Primary Hyperparathyroidism (PHPT)

Primary Hyperparathyroidism (PHPT) is thought to be the second most common cause of pathologic hypercalcemia (increased serum calcium) in dogs. It is caused by an inappropriate secretion of parathyroid hormone (PTH) by autonomously functioning "chief" cells within the parathyroid glands in the neck of the dogs. There are four parathyroid glands, two on each side. They are called "para" thyroid because they are next to the thyroid gland but functionally they are distinct from the thyroid gland. Thus parathyroid diseases are not connected to thyroid diseases like hyperthyroidism or hypothyroidism. Parathyroid hormone is an important factor in maintaining calcium homeostasis (normal calcium concentrations and function).

When secreted, PTH causes an increase in serum calcium via bone resorption, decreased calcium excretion from the kidneys and increased vitamin D metabolism. Vitamin D causes increased absorption of calcium from the intestines. Parathyroid hormone secretion by parathyroid chief cells is normally regulated tightly by serum ionized (or unbound) calcium. Normally, if the concentration of ionized calcium rises, PTH secretion is then decreased through negative feedback, allowing the calcium concentration to go back to normal. In PHPT, PTH secretion persists despite increased calcium concentrations resulting in many instances in severe, life threatening hypercalcemia. Most cases of canine PHPT (80-85%) result from a solitary parathyroid adenoma, a solitary benign tumor of the parathyroid gland. Parathyroid hyperplasia affecting multiple glands occurs in most of the remaining cases while the malignant parathyroid carcinoma is considered to be very rare in dogs.

This condition affects middle aged to older dogs with a mean age of approximately 10.5 years, according to the existing veterinary literature without taking specific breeds into account. In the early phases of this disease, the affected dogs tend to be relatively free of symptoms except for increased drinking and urination, in addition to a gradual onset of weakness, lethargy, shaking and sometimes weight loss. Many have concurrent calcium containing urolithiasis (bladder or kidney stones). As the hypercalcemia becomes more profound, organ damage occurs, including possible damage to bones as well as severe kidney damage. If the hypercalcemia is left untreated, this secondary kidney disease can eventually lead to severe renal failure and death.

The diagnosis of PHPT is based on the finding of normal or increased serum PTH concentrations in the presence of increased total and ionized calcium concentrations. The enlarged parathyroid gland or glands can frequently also be identified with cervical (neck) ultrasound. The treatment of PHPT involves surgical removal or ultrasound guided chemical ablation or heat ablation of the affected parathyroid gland. When performed early in the disease, prior to the onset of renal failure and with appropriate post operative care and monitoring, this treatment is typically very successful with a relatively low percentage of reoccurrence.

PHPT testing is now being performed by the Molecular Diagnostic section at the Animal Health Diagnostic Center. Please see the links below for sample submission instructions and forms:

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