Definition

Absolute or relative deficiency of parathyroid hormone secretion leading to hypocalcemia.

Pathophysiology

• Dogs-most commonly idiopathic immune-mediated parathyroiditis.
• Cats-most commonly iatrogenic secondary to damaged or removed parathyroid glands during thyroidectomy for hyperthyroidism; idiopathic atrophy and immune-mediated parathyroiditis also seen (uncommon).

Systems Affected

• Cardiovascular-ECG changes and bradycardia caused by altered neuromuscular activity.
• Gastrointestinal-anorexia and vomiting of unknown cause, possibly changes in gastrointestinal muscular activity.
• Nervous/Neuromuscular-seizures, tetany, ataxia, and weakness caused by increased neuromuscular activity resulting from diminished neuronal membrane stability.
• Ophthalmic-posterior lenticular cataracts of unknown cause.
• Renal/Urologic-PU/PD of unknown cause.
• Respiratory-panting caused by neuromuscular weakness and anxiety associated with neurologic and neuromuscular changes.

Genetics

Unknown

Incidence/Prevalence

• Dog-uncommon; exact prevalence not reported.
• Cat-common in bilaterally thyroidectomized cats (10–82% of patients, depending on surgical technique and surgical skill); spontaneous occurrence rare (nine cases reported in literature, five additional cats discussed in Canine and Feline Endocrinology and Reproduction textbook-see “Suggested Reading”).

Signalment

Signs

Causes

See “Pathophysiology”

Risk Factors

• Dogs-N/A
• Cats-bilateral thyroidectomy for hyperthyroidism

Differential Diagnosis

The main problems associated with hypoparathyroidism, which must be differentiated from other disease processes, are seizures, weakness, and muscle trembling, twitching, and fasciculations.

CBC/Biochemistry/Urinalysis

• Results of hemogram and urinalysis usually normal; these tests are performed to rule-out other differential diagnoses.
• Hypocalcemia (usually <6.5 mg/dL) and normal or mild to moderate hyperphosphatemia.
• Evaluate serum albumin carefully in all patientswithhypocalcemia;hypoalbuminemia is the most common cause of hypocalcemia.
• The only other intrinsic disease process that reduces serum calcium and raises serum phosphorus is renal failure, which is easily distinguished from hypoparathyroidism by the presence of azotemia. Highly concentrated phosphate enema solutions can also cause hyperphosphatemia and hypocalcemia.

Other Laboratory Tests

Serum PTH determination-demonstrates undetectable or very low concentration of PTH; patients with other processes causing hypocalcemia (e.g., renal failure) have a normal-to-high concentration of PTH.

Imaging

Radiography and ultrasonography are normal.

Diagnostic Procedures

• ECG changes seen in patients with hypocalcemia include prolongation of the ST and QT segments; sinus bradycardia and wide T waves; T wave alternans is occasionally seen.
• Cervical exploration reveals absence or atrophy of the parathyroid glands.

Pathologic Findings

• Dogs-normal tissue with mature lymphocytes, plasma cells, and fibrous connective tissue along with chief cell degeneration.
• Cats-parathyroid gland atrophy is more common, although histopathologic findings similar to those in dogs are found in cats.

Appropriate Health Care

• Hospitalize for medical management of hypocalcemia until clinical signs of hypocalcemia are controlled and serum calcium concentration is >7 mg/dL.
• See Hypocalcemia for emergency inpatient management and appropriate fluid therapy.

Nursing Care

Usually not required; hydration and nutritional support if anorexic.

Activity

Normal

Diet

Avoid calcium-poor diets; for dogs, puppy diets generally higher in calcium than adult dog food.

Client Education

• Naturally occurring primary hypoparathyroid will require lifelong therapy and monitoring.
• Most cases of iatrogenic hypoparathyroidism (e.g., bilateral thyroidectomy) will recover over days to months and only require transient management and monitoring.

Surgical Considerations

None

Drug(s) Of Choice

Contraindications

See Hypocalcemia

Precautions

All calcium preparations given orally can cause gastrointestinal disturbances; calcium carbonate may be less irritating because of its high calcium availability and lower dosage requirement.

Possible Interactions

• Injectable calcium solutions are reportedly incompatible with tetracycline drugs, cephalothin, methylprednisolone sodium succinate, dobutamine, metoclopramide, and amphotericin B.
• Thiazide diuretics used in conjunction with large doses of calcium may cause hypercalcemia.
• Patients on digitalis are more likely to develop arrhythmias if calcium is administered intravenously.
• Calcium administration may antagonize effects of calcium channel blocking agents (e.g., diltiazem, verapamil, nifedipine, and amlodipine).

Alternative Drug(s)

None

Patient Monitoring

• Hypocalcemia and hypercalcemia are both concerns with long-term management.
• Once serum calcium is stable and normal, assess serum calcium concentration monthly for the first 6 months, then every 2–4 months; goal is to maintain serum calcium between 8 and 10 mg/dL.
• Inform clients about clinical signs of hypo- and hypercalcemia.

Prevention/Avoidance

N/A

Possible Complications

• Hypocalcemia
• Hypercalcemia, which can lead to nephrocalcinosis and kidney injury (see Hypercalcemia)

Expected Course and Prognosis

• With close monitoring of serum calcium and client dedication, the prognosis for long-term survival is excellent.
• Adjustments in vitamin D and oral calcium administration can be expected during the course of management, especially during the initial 2–6 months.
• Cats with hypoparathyroidism secondary to thyroidectomy usually require only transient treatment because they typically regain normal parathyroid function within 4–6 months, often within 2–3 weeks.

Associated Conditions

Excess muscular activity can lead to hyperthermia, which may necessitate treatment.

Age-Related Factors

N/A

Zoonotic Potential

None

Pregnancy/Fertility/Breeding

Hypocalcemia can lead to weakness and dystocia.

See Also

• Hypercalcemia
• Hyperthyroidism
• Hypocalcemia

Abbreviations

• Ca = calcium
• ECG = electrocardiography
• PTH = parathyroid hormone
• PU/PD = polyuria and polydipsia

Internet Resources

http://www.vet.uga.edu/VPP/CLERK/mcfarland/index.php.

Client Education Handout Available Online