PROFILE
- Primary hyperparathyroidism (PHPT) and primary hypoparathyroidism are the most common primary parathyroid gland diseases.
- Two external parathyroid glands lie outside the thyroid capsule, and two internal parathyroid glands are embedded within the thyroid parenchyma.1
- Parathyroid glands synthesize and secrete parathyroid hormone (PTH) from chief cells.2
- PTH increases plasma calcium concentration by mobilizing calcium from bone, increasing renal calcium reabsorption, and promoting formation of calcitriol, which increases intestinal calcium absorption.
- PTH promotes phosphaturia by decreasing renal phosphorus reabsorption.
- Parathyroid gland diseases are characterized by abnormalities in serum calcium and phosphorus concentrations.
- Clinical signs are frequently secondary to serum calcium abnormalities.
- Parathyroid glands may also be affected secondary to other disease states (eg, renal secondary hyperparathyroidism).
Related Article: Anesthesia for Parathyroid Disease
Definition & Pathophysiology3,4
- Excessive PTH production from autonomously functioning chief cells, usually in a single parathyroid adenoma
- Parathyroid carcinoma, hyperplasia, or multiple parathyroid adenomas are possible but rare.
Systems
- Urinary, neuromuscular, and GI signs are possible.
Incidence & Prevalence
- Accounts for ~13% of dogs with ionized hypercalcemia5
- PHPT is caused by adenoma of the parathyroid gland (75%–85% of cases), hyperplasia (5%–15% of cases), and carcinoma (5%–10% of cases).4,6,7
- Rarer in cats8
- More common causes of hypercalcemia in cats include renal failure and idiopathic hypercalcemia (see Table)
Related Article: Polyuria, Polydipsia, & Hypercalcemia
Signalment
Breed Predilection
- Autosomal dominant inheritance causes increased prevalence in keeshonds,9 but PHPT should be considered as a differential for any dog or cat with hypercalcemia.
Age & Range
- Middle-aged to geriatric dogs
Sex Predilection
- No known predilection
Related Article: Hypercalcemia in Dogs and Cats
Clinical Signs3,6
- Mainly attributable to hypercalcemia, which may be found incidentally:
- Up to 35% of patients show no clinical signs.
- Affected patients often appear healthier than those with other causes of hypercalcemia (eg, lymphoma).
- Causes(s) and approximate frequency of clinical signs:
- Polyuria/polydipsia from decreased renal tubular response to antidiuretic hormone (50%–60%)
- Hematuria, stranguria, pollakiuria, urinary tract infection, and urinary tract obstruction (50%)
- Calcium phosphate or oxalate urolithiasis may result from calciuria and phosphaturia.
- Dilute urine-specific gravity predisposes patients to infection.
- Lethargy, muscle wasting, and stiffness from decreased neuromuscular tissue excitability (40%–45%)
- Inappetence (37%), vomiting (13%), and constipation (6%) caused by decreased excitability and motility of GI smooth muscle
Physical Examination
- Patients may appear lethargic or weak.
- A cervical mass may be palpated in cats.
- A palpable parathyroid mass is rare in dogs.
- A palpated mass could represent an alternative cause of hypercalcemia (eg, thyroid carcinoma).
- Examination should not identify any finding consistent with other causes of hypercalcemia (eg, lymphadenopathy).
DIAGNOSIS
Definitive
- Histologic examination of parathyroid gland mass(es) following successful surgical removal/ablation and resolution of hypercalcemia
- Plasma PTH can be measured to support diagnosis.
Differentials
- Differential diagnoses for hypercalcemia can vary (see Table)
Laboratory Findings
- Chemistry panel: total hypercalcemia, decreased or borderline-low serum phosphorus concentration, possible azotemia
- Ionized hypercalcemia in >90% of cases3
- Urinalysis: frequent hyposthenuria or isosthenuria
- Crystalluria, bacteriuria, hematuria, and pyuria are possible.