Familial Hypocalciuric Hypercalcemia

At a Glance

Familial hypocalciuric hypercalcemia is an unusual, usually benign condition that causes chronically elevated serum calcium and reduced calcium excretion.

It is typically caused by an abnormal set-point for parathyroid hormone (PTH) secretion in the calcium sensing receptor (CASR). The CASR is expressed primarily in the parathyroid and maintains calcium activity ("ionized calcium") at a constant level very close to 1.25 mM by controlling PTH secretion. Although rapid changes in serum calcium activity cause serious metabolic problems, in familial hypocalciuric hypercalcemia there is slow, chronic calcium elevation, allowing patients to acclimate to it. Therefore, patients do not generally have symptoms or require treatment. However, patients with familial hypocalciuric hypercalcemia may be more sensitive than normal patients to dietary calcium or vitamin D toxicity, so these patients are normally not candidates for the ubiquitous calcium and vitamin D supplementation.

The disorder can rarely be acquired as an autoimmune syndrome.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

In asymptomatic hypercalcemia, measure urine calcium, serum PTH and serum magnesium. Genetic analysis of family members may also be useful. (Table 1)

Table 1.

Test Results Indicative of the Disorder
Urinary calcium excretion Serum Calcium Serum PTH Serum Magnesium
Reduced Elevated Usually low Generally elevated, but not severely

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

There are no known factors or medications that affect laboratory results. The main concern is making the mistake of associating hypercalcemia with a benign familial condition when there is a more serious, treatable cause.

What Lab Results Are Absolutely Confirmatory?

Genetic analysis of the CASR showing a point mutation can be done; familial occurrence in an autosomal dominant pattern is good confirmation. Although CASR is diagnostic, there is no treatment for familial hypocalciuric hypercalcemia, so its utility is questionable.

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