In the preceding article, we discussed the pathophysiology of primary hypoparathyroidism and resulting hypocalcemia. The clinical
signs of hypocalcemia are the same regardless of the underlying cause. In this article, we review the differential diagnoses
in patients with hypocalcemia, the diagnostic testing to investigate clinical signs consistent with hypocalcemia, and the
treatment of primary hypoparathyroidism.
DIFFERENTIAL DIAGNOSES
The various differential diagnoses in patients with hypocalcemia can be organized based on the degree of hypocalcemia and
the prevalence of the underlying disease causing hypocalcemia. An important cause for a falsely low serum calcium concentration
is laboratory error or improper sample anticoagulant (addition of EDTA, which chelates calcium).1
Common causes of low serum total calcium include hypoalbuminemia, renal failure, puerperal tetany, and acute pancreatitis.1,2 Less frequently, hypocalcemia occurs with ethylene glycol intoxication, administration of phosphate-containing enemas or
intravenous sodium bicarbonate, soft tissue trauma, rhabdomyolysis, and primary hypoparathyroidism.1,2 Other causes of hypocalcemia include rapid intravenous administration of phosphates, dilution when calcium-free intravenous
fluids are administered, intestinal malabsorption, starvation, citrated blood or plasma transfusions, hypovitaminosis D, thyroid
tumors, cervical trauma or surgery, hypomagnesemia, aminoglycoside intoxication, and nutritional secondary hyperparathyroidism.1-3
Causes of hypocalcemia that have been identified in people but have not yet been clearly established in cats or dogs include
pseudohypoparathyroidism (end-organ resistance to parathyroid hormone [PTH]), drug-induced hypoparathyroidism (most commonly
from chemotherapeutic agents), sepsis, primary or metastatic bone tumors, hypercalcitoninism, and 131 I radiation damage.1,2,4
Primary hypoparathyroidism may be due to the absence or destruction of the parathyroid gland (as with surgical removal or
immune-mediated disease, respectively). Transient hypoparathyroidism may occur with rapid correction of long-standing hypercalcemia
(hypercalcemia causing temporary atrophy of the gland), as can occur with surgical removal or ablation of a parathyroid adenoma
causing primary hyperparathyroidism.1,2 Thyroid surgery in both dogs and cats may result in removal or damage to the parathyroid glands. Thyroidectomy is the most
common cause of hypoparathyroidism in cats.1,3 Parathyroid agenesis has been reported in a dog1 and was suspected in a cat.5
DIAGNOSIS
A serum chemistry profile may reveal results that are indicative of primary hypoparathyroidism. Primary hypoparathyroidism
can be definitively diagnosed by measuring serum pth and ionized calcium concentrations.
Biochemical profile
In addition to ionized hypocalcemia, animals with hypoparathyroidism have relative or absolute hyperphosphatemia.1 In a case series of 37 dogs with hypoparathyroidism, the serum phosphorus concentrations were greater than the serum calcium
concentrations in every case.1 Young animals normally have slightly higher phosphorus concentrations, and most laboratories include these values when establishing
their reference ranges, resulting in higher reference ranges for phosphorus concentration.1,3 However, anorectic hypoparathyroid dogs may have relative phosphate depletion, which may or may not be reflected in the
serum phosphorus concentration.1 The combination of hypocalcemia and hyperphosphatemia, with normal serum albumin, blood urea nitrogen, and creatinine concentrations,
is indicative of primary hypoparathyroidism.1,2
Parathyroid hormone measurement
In the past 15 years, human PTH assays have been validated for use in dogs and cats.1,4,6-8 Before a validated PTH assay became available for use in animals, a histologic examination was used to confirm the diagnosis
of primary hypoparathyroidism.6,9 With the advent of a reliable PTH assay, biopsy is no longer necessary to confirm hypoparathyroidism.
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