Urine Calcium: Laboratory Measurement and Clinical Utility

Kevin F. Foley, PhD, DABCC; Lorenzo Boccuzzi, DO

Disclosures

Lab Med. 2010;41(11):683-686. 

In This Article

Hypocalciuria

Hypocalciuria is often mistakenly due to incomplete collection (a random sample thought to be an aliquot from a 24-hour specimen). Some drugs can decrease urine calcium, including thiazide diuretics, benzothiadiazide diuretics (like chlorthalidone), and estrogen. Decreased urine calcium is also seen in hypoparathyroidism, pseudohypoparathyroidism (a lack of response to PTH rather than decreased secretion of PTH), rickets, hypothyroidism, steatorrhea, and nephrosis. Another cause of low urine calcium is familial hypocalciuric hypercalcemia (FHH), also known as familial benign hypercalcemia. This disease is often initially misdiagnosed as hyperparathyroidism, but unlike hyperparathyroidism, FHH will not resolve with parathyroidectomy. Familial hypocalciuric hypercalcemia is now known to be caused by various autosomal-dominant loss-of-function mutations in the gene coding for the CASR.[4] The CASR responds to serum calcium and mediates feedback inhibition of PTH release. A loss-of-function mutation results in a rising of the calcium threshold that triggers reduction of PTH secretion. Serum concentration of calcium is thus maintained at higher levels since more calcium is needed to trigger negative feedback. This results in hypercalcemia, and since more calcium is sequestered in the serum, hypocalciuria often results. It is important to note that hyperparathyroidism and FHH can show elevated serum PTH levels. There are also known cases in which FHH patients have hypercalciuria.[5]

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