Ionized Calcium

The measurement of ionized calcium appears to have a major role in the identification of symptomatic patients with PHPT with minimal, intermittent, or no elevation of the total calcium.6,7 23 Ionized calcium is measured with a calcium-selective ion flow-through electrode system.24 This system for measuring ionized calcium was first introduced in 1967 and has since undergone several design changes that have resulted in improved precision.

Previous articles and continuing experience indicate that the serum ionized calcium is superior to total calcium in detecting PHPT in patients with intermittent, minimal, or no elevation of the total calcium. Ionized calcium is of no added benefit for diagnostic purposes in patients with elevated total serum calcium. Hypersecretion of PTH increases serum calcium by increasing the tubular resorption of calcium, increasing the net bone resorption and, to a lesser extent, increasing the intestinal absorption of calcium. One can justifiably question why total calcium is not increased in all patients with PHPT, assuming a normal serum albumin and no pancreatitis, increased phosphate intake, or hypo-magnesemia, factors known to cause a decrease in the total serum calcium. Some authors have attributed the normal total serum calcium in normocalcemic hyperparathyroidism to an increased ratio of ionized and ultrafiltrable calcium to total calcium compared to normal individuals. Elevated serum PTH has been postulated to decrease the binding of calcium to protein and therefore increases ionized calcium at the expense of the protein-bound fraction.6-23'25 An alternative explanation postulated for normocalcemia in patients with PHPT is a generalized resistance to the action of PTH on bone and kidney.26

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