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Published online by Cambridge University Press: 12 January 2010
Both hypercalcemia and hypocalcemia may be associated with life-threatening cardiac arrhythmias as well as morbidity affecting other organ systems. Effective treatment is available and clinicians should be alert to abnormalities in serum calcium, which are present in more than 2% of hospitalized patients. Furthermore, both hypercalcemia and hypocalcemia suggest significant underlying pathology, and efforts to diagnose and treat these conditions should be instituted.
Adult humans contain more than 1 kg of calcium, of which over 99% is skeletal and dental and only 0.1% is in extracellular fluids. About half the calcium in serum is bound to protein, primarily albumin. Decreases in serum albumin are accompanied by decreases in calcium (a drop of 1 g/dl of albumin lowers the calcium by about 0.8 mg/dl). Several calcium determinations and measurement of ionized (physiologically active) calcium levels may be needed to accurately assess calcium status.
Serum ionized calcium levels are tightly controlled by the interplay of parathyroid hormone, calcitonin, and 1,25-dihydroxycholecalciferol (1,25-[OH]2D3). Parathyroid hormone is synthesized in the parathyroid glands and, after cleavage of precursor molecules, is released into the circulation as an 84-amino-acid polypeptide and small fragments.
The amino-terminal 1–34 amino acids compose the biologically active portion of the molecule. Highly specific immunoradiometric assays are available that measure the intact hormone, permitting accurate diagnosis. Parathyroid hormone release is primarily controlled by serum calcium levels, although modest hypomagnesemia also evokes a parathyroid hormone response, whereas severe hypomagnesemia impairs release.
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