Hypocalcemia

For further information, see CMDT Part 21-07: Hypocalcemia

Often mistaken as a neurologic disorder
Decreased serum parathyroid hormone (PTH), vitamin D, or magnesium levels
Despite a low total serum calcium, calcium metabolism is likely normal if ionized calcium level is normal

Increased excitation of nerve and muscle cells, primarily affects the neuromuscular and cardiovascular systems
Hypoalbuminemia is most common cause of low total serum calcium
When serum albumin concentration is < 4 g/dL (40 g/L), serum Ca²⁺ concentration is reduced by 0.8–1 mg/dL (0.20–0.25 mmol/L) for every 1 g/dL (10 g/L) of albumin
The most accurate measurement of serum calcium is the ionized calcium concentration
True hypocalcemia (decreased ionized calcium) implies insufficient action of PTH or active vitamin D
Important causes of hypocalcemia are listed in Table 21–6
The most common cause of hypocalcemia is advanced CKD
Magnesium depletion reduces both PTH release and tissue responsiveness to PTH, causing hypocalcemia
Hypocalcemia in pancreatitis is a marker of severe disease
Elderly hospitalized patients with hypocalcemia and hypophosphatemia, with or without an elevated PTH level, are likely vitamin D deficient

Table 21–6.Causes of hypocalcemia.

Table 21–6. Causes of hypocalcemia.

Decreased intake or absorption

Malabsorption

Small bowel bypass, short bowel

Vitamin D deficit (decreased absorption, decreased production of 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D)

Increased loss

Alcoholism

Chronic kidney disease

Diuretic therapy

Endocrine disease

Hypoparathyroidism (genetic, acquired; including hypomagnesemia and hypermagnesemia)

Post-parathyroidectomy (hungry bone syndrome)

Pseudohypoparathyroidism

Calcitonin secretion with medullary carcinoma of the thyroid

Familial hypocalcemia

Associated diseases

Pancreatitis

Rhabdomyolysis

Septic shock

Physiologic causes

Decreased serum albumin¹

Decreased end-organ response to vitamin D

Hyperphosphatemia

Aminoglycoside antibiotics, plicamycin, loop diuretics, foscarnet

¹Ionized calcium concentration is normal.

Spasm of skeletal muscle causes cramps and tetany
Laryngospasm with stridor can obstruct the airway
Convulsions, paresthesias of lips and extremities, and abdominal pain
Chvostek sign (contraction of the facial muscle in response to tapping the facial nerve)
Trousseau sign (carpal spasm occurring after occlusion of the brachial artery by a blood pressure cuff) is usually readily elicited
In chronic hypoparathyroidism, cataracts and calcification of basal ganglia may appear
QT prolongation predisposes to ventricular arrhythmias

Serum calcium concentration is low (< 8.5 mg/dL or < 2.21 mmol/L)
In true hypocalcemia, the ionized serum calcium concentration is low (< 4.6 mg/dL or < 1.15 mmol/L)
Serum phosphate is usually elevated in hypoparathyroidism or advanced CKD, whereas it is suppressed in early CKD or vitamin D deficiency
Serum magnesium concentration is commonly low
In respiratory alkalosis, total serum calcium is normal but ionized calcium is low

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