Hypoparathyroidism

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Punctate cataract in a dog with primary hypoparathyroidism[1]

Hypoparathyroidism is an uncommon endocrine disease characterized by a physical or physiological inactivity of parathyroid hormone (PTH) by the parathyroid gland.

Hypoparathyroidism may be a primary disease or secondary to other systemic disease(s). With primary hypoparathyroidism, the common cause is idiopathic (immune-mediated) lymphocytic parathyroiditis[2].

Parathyroid hormone (parathormone) is secreted by the chief cells of the parathyroid glands and causes an increase in blood concentration of ionized calcium, whereas calcitonin (a hormone produced by the parafollicular cells (C cells) of the thyroid gland) acts to decrease calcium concentration.

The contrary condition, hyperparathyroidism also occurs frequently in dogs.

With hypoparathyroidism, a predisposition has been noted with middle-aged female Miniature Schnauzers, St. Bernards[3], Chihuahua, German Shepherds and Jack Russell Terriers[4].

This condition can occur as a result of:

- lymphocytic parathyroiditis (idiopathic hypoparathyroidism)[5]
  • Infections
  • Protein-losing enteropathy and secondary hypomagnesemia[6]
  • Chronic hypercalcemia
  • Tumor lysis syndrome associated with chemotherapy[7] or surgical lumpectomy[8]

Clinical symptoms is affected dogs develop over 1 - 4 weeks due to progressive hypocalcemia, and present initially with muscle twitching, polyuria, polydipsia, anorexia, and sometimes periocular alopecia.

As the condition worsens, mental depression, seizures, muscle tremors and fasciculations, stiff gait, tetany, muscle cramping, behavioural changes and cataracts (posterior lenticular cataract formation secondary to hypocalcemia)[9] may develop.

A tentative diagnosis can be made from blood results which usually show severe hypocalcemia (less than 6.5 mg/dl; normal 9 - 11.4), mild hyperphosphatemia (due to increased renal tubular reabsorption) and reduced intact parathyroid hormone levels[10].

Radiography may elucidate calcification of microvasculature, intracerebral calcification, osteopenia and ligamentous ossification.

Electrocardiographys often exhibit prolonged QT intervals due to a ST-segment prolongation.

In the early stages of immune-mediated lymphocytic parathyroiditis in dogs, there is infiltration of the gland with lymphocytes and plasma cells and nodular regenerative hyperplasia of remaining chief cells. Later, the parathyroid gland is replaced by lymphocytes, fibroblasts, and capillaries, with only an occasional viable chief cell.

A response to therapy may also suggest a diagnosis of primary hypoparathyroidism.

A differential diagnosis would include neoplasia and milk fever (perparturient hypocalcemia; usually has concurrent hypophosphatemia).

Treatment involves addressing the acute hypocalcemia with parenteral calcium gluconate (10 mL of 10% calcium gluconate in 250 mL of 0.9% saline administered at 2.5 mL/kg/hr for 8 – 12 hours) and intravenous anticonvulsants.

Chronic therapy included oral vitamin D analogues (calcitriol ≥25,000 – 50,000 U/day)[11], dihydrotachysterol and dietary calcium supplementation, with monitoring of blood calcium to minimize risk of secondary vitamin D toxicosis[12] and calcinosis cutis[13].

Fanconi's syndrome has been reported as a complication of canine hypoparathyroidism[14].

Once the blood calcium has returned to normal, substantially lower doses of vitamin D are indicated for longterm maintenance; in some dogs, only dietary calcium supplementation is required for longterm stabilization.

Levels of parathyroid hormone may remain suppressed for some time post-therapy, especially in older dogs.

References

  1. Vet Grad
  2. Sherding RGet al (1980) Primary hypoparathyroidism in the dog. J Am Vet Med Assoc 176(5):439-444
  3. Jones BR & Alley MR (1981) Primary idiopathic hypoparathyroidism in St. Bernard dogs. N Z Vet J 33(6):94-97
  4. Russell NJ et al (2006) Primary hypoparathyroidism in dogs: a retrospective study of 17 cases. Aust Vet J 84(8):285-290
  5. Bruyette DS & Feldman EC (1988) Primary hypoparathyroidism in the dog. Report of 15 cases and review of 13 previously reported cases. J Vet Intern Med 2(1):7-14
  6. Bush WW et al (2001) Secondary hypoparathyroidism attributed to hypomagnesemia in a dog with protein-losing enteropathy. J Am Vet Med Assoc 219(12):1732-1734
  7. Horn B & Irwin PJ (2000) Transient hypoparathyroidism following successful treatment of hypercalcaemia of malignancy in a dog. Aust Vet J 78(10):690-692
  8. Henderson RA et al (1991) Development of hypoparathyroidism after excision of laryngeal rhabdomyosarcoma in a dog. J Am Vet Med Assoc 198(4):639-643
  9. Higgins MA & Rossmeisl JH (2005) What is your neurologic diagnosis? Hypoparathyroidism. J Am Vet Med Assoc 226(5):699-701
  10. Torrance AG & Nachreiner R (1989) Intact parathyroid hormone assay and total calcium concentration in the diagnosis of disorders of calcium metabolism in dogs. J Vet Intern Med 3(2):86-89
  11. Diquélou A et al (2005) Hypocalcaemia due to nutritional calcium deficiency and hypoparathyroidism in an adult dog. Vet Rec 156(2):45-48
  12. Mellanby RJ et al (2005) Hypercalcaemia in two dogs caused by excessive dietary supplementation of vitamin D. J Small Anim Pract 46(7):334-338
  13. Schaer M et al (2001) Severe calcinosis cutis associated with treatment of hypoparathyroidism in a dog. J Am Anim Hosp Assoc 37(4):364-369
  14. Freeman LM et al (1994) Fanconi's syndrome in a dog with primary hypoparathyroidism. J Vet Intern Med 8(5):349-354