7.12 Calcium Disorders
Key Features
Hypercalcemia can result when too much calcium enters the extracellular fluid or when there is insufficient calcium excretion from the kidneys.
Hypocalcemia is less frequent than hypercalcemia.
Hypocalcemia occurs in patients with renal failure, vitamin D deficiency, magnesium deficiency, acute pancreatitis, and with hypoparathyroidism and pseudohypoparathyroidism.
About 99% of the body′s calcium is found in the bones. The remaining 1% is found in the extracellular fluid. Of this 1%, ~ 40% of calcium is bound to albumin, and 50% is in the free (unbound, active, or ionized) form. A basic metabolic panel and a comprehensive metabolic panel measure the total calcium levels (bound plus unbound), although it is the free (unbound) form that is most important.
Control of Calcium Metabolism
Calcium is absorbed from the gut, stored in the bone, and excreted by the kidneys. Calcium levels are affected by the following:
Parathyroid hormone (PTH): Decreased serum calcium levels lead to an increase in PTH, which in turn causes a release of bone calcium stores and decreased renal excretion of calcium.
Vitamin D: PTH stimulates conversion of 25(OH)D into its active form [1,25-(OH)2D], which in turn increases calcium and phosphate absorption from the gut.
Calcitonin: Calcitonin is synthesized in the C cells of the thyroid and causes a decrease in plasma calcium and phosphate levels.
Hypocalcemia
Etiology
Hypoparathyroidism: The most common cause is iatrogenic (surgery); see Chapter 7.11.
Pseudohypoparathyroidism: An inherited disorder that is caused by resistance to PTH by target organs. Patients present in childhood. It is associated with hypocalcemia, hyperphosphatemia, and elevated PTH levels.
Renal failure
Vitamin D deficiency
Hypomagnesemia
Acute pancreatitis
Hyperphosphatemia
Clinical
Neuromuscular irritability: Patients may present with tingling, paresthesias in fingers and periorally, tetany, carpopedal spasm, seizures, irritability, and confusion.
Chvostek′s sign: Gentle tapping over the facial nerve causes twitching of facial muscles.
Trousseau′s sign: Carpopedal spasm after blood pressure cuff is inflated above patient′s systolic blood pressure for 3 minutes.
Cardiac manifestations: electrocardiogram changes—prolonged QT interval, heart failure, and arrhythmias
Other manifestations include subcapsular cataracts, dry flaky skin, and brittle nails.
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