Laboratory Studies
Parathyroid hormone
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Primary hypoparathyroidism is defined by a low concentration of PTH with a concomitant low calcium level.
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In pseudohypoparathyroidism, the serum PTH concentration is elevated as a result of resistance to PTH caused by mutations in the PTH receptor system.
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In secondary hypoparathyroidism, the serum PTH concentration is low and the serum calcium concentration is elevated.
Calcium
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The calcium ion is highly bound to protein. A total calcium level cannot be interpreted without a total protein or albumin level.
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Hypoalbuminemia causes a drop in total calcium concentration, but the ionized fraction may be within the reference range. Elevated protein states, such as multiple myeloma and paraproteinemias, may cause an elevation of the total calcium concentration, but the ionized fraction may be within the reference range.
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Conversely, in the presence of albumin or protein excess, low ionized calcium levels with reference range levels of total calcium are possible. Likewise, if the patient is hypoalbuminemic, high ionized calcium levels with a reference range level of total calcium are possible.
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Measurement of ionized calcium concentration in the plasma is ideal; however, it is not readily available in many places.
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The relationship between total serum calcium and albumin is defined by the following simple rule: the serum total calcium concentration falls by 0.8 mg/dL for every 1-g/dL fall in serum albumin concentration. This rule assumes that normal albumin equals 4.0 g/dL and normal calcium is 10.0 mg/dL.
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Alkalosis causes ionized calcium to bind to albumin more strongly. This causes a decrease in the ionized calcium and may trigger symptoms of hypocalcemia.
Measurement of 25-hydroxy vitamin D: This measurement is important to exclude vitamin D deficiency as a cause of hypocalcemia.
Serum magnesium: Hypomagnesemia may cause PTH deficiency and subsequent hypocalcemia. Exclude it in any patient with primary hypoparathyroidism.
Serum phosphorus: PTH is a phosphaturic hormone. In its absence, phosphorus levels in the blood rise.
