Ophthalmologic manifestations

In people, cats, and dogs, hypoparathyroidism can be associated with the development of bilateral, small, punctate-to-linear cataracts in the anterior and posterior cortical subcapsular region.^1,3,4,15 The mechanism of cataract formation is unknown, and vision is not typically impaired if treatment is initiated early.^1,15 The cataracts are slowly progressive if the hypoparathyroidism is left untreated.¹

In patients with clinical signs consistent with hypocalcemia, perform a serum chemistry profile. Measure the total serum calcium, phosphorus, blood urea nitrogen, creatinine, and albumin concentrations. Ideally, a complete blood count and urinalysis should also be performed. In most laboratories, the lower end of the reference range of total calcium is 9.5 mg/dl in dogs and 9 mg/dl in cats.¹ Dogs with untreated hypoparathyroidism often have serum total calcium concentrations consistently below 6.5 mg/dl.^1,2

When the total calcium concentration is below the reference range, two factors must be considered. First, a common cause of hypocalcemia is sampling or laboratory error. So confirm hypocalcemia by obtaining a second blood sample to repeat the total calcium concentration or measure an ionized calcium concentration.¹ Second, consider the albumin concentration. Hypoalbuminemia will lower the total calcium concentration in the blood, but the ionized fraction will likely be normal.^1,8 Formulas for corrected calcium concentrations exist, but their accuracy remains controversial.¹ The most common formula for corrected calcium is corrected calcium is equal to the measured total calcium concentration (mg/dl) – albumin concentration (g/dl) + 3.5.¹ With the availability of equipment to measure ionized calcium concentrations, the use of corrected formulas is falling out of favor.

In dogs with hypoparathyroidism, the albumin, blood urea nitrogen, and creatinine concentrations are typically normal.^1,2,5 Evaluation of total serum magnesium may be warranted. In the case series of 37 dogs, 23 had total serum magnesium concentration evaluated.¹ Nineteen of the 23 were normal and four were decreased. However, total serum magnesium concentration may not accurately reflect the biologically active component (ionized magnesium).

In any patient with hypocalcemia, evaluate the ionized fraction.⁸ Ionized calcium concentrations below 1.1 mmol/L in dogs and below 1 mmol/L in cats are diagnostic for hypocalcemia.¹ Whole blood, plasma, or serum samples are appropriate as long as they are collected under anaerobic conditions.^4,11 If a plasma sample is processed within three days, refrigeration of the sample is sufficient. If there is a delay in processing, the sample should be frozen.¹ Before collecting the samples, contact your laboratory for guidelines on handling samples. It may be prudent to simultaneously request a PTH concentration.

CONCLUSION

To recognize primary hypoparathyroidism, you must have a thorough understanding of the interaction of PTH and calcitriol and the effect that both have on serum ionized calcium concentration. Neurologic and neuromuscular clinical signs predominate. Once these signs are noted, you should perform diagnostic testing. Once you have definitively diagnosed primary hypoparathyroidism, begin appropriate therapy. The next article discusses the differential diagnoses in dogs and cats with hypocalcemia, additional diagnostic testing, and treatment of primary hypoparathyroidism.

Beth L. McElravy, DVM
Jill D. Brunker, DVM, DACVIM
Department of Small Animal Internal Medicine
College of Veterinary Medicine
Oklahoma State University
Stillwater, OK 74078

REFERENCES

1. Feldman EC, Nelson RW. Hypocalcemia and primary hypoparathyroidism. In: Canine and feline endocrinology and reproduction. 3rd ed. Philadelphia, Pa: Elsevier Science, 2004;716-742.

2. Feldman EC. Disorders of the parathyroid glands. In: Ettinger SJ, Feldman EC, eds. Textbook of veterinary internal medicine: diseases of the dog and cat. 6th ed. St. Louis, Mo: Elsevier Saunders, 2005;1508-1535.