Familial hypocalciuric hypercalcemia

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Familial hypocalciuric hypercalcemia
Classification and external resources
ICD-10 E83.5
OMIM 145980 145981 600740
DiseasesDB 1326
MeSH D006934

Familial hypocalciuric hypercalcemia is a condition that can cause hypercalcemia, a serum calcium level typically above 10.2 mg/dL. It is also known as familial benign hypocalciuric hypercalcemia (FBHH) where there is usually a family history of hypercalcemia which is mild, a urine calcium to creatinine ratio <0.01, and urine calcium <200 mg/day.

Causes[edit]

Types include:

Name OMIM Locus Gene
HHC1 145980 3q13.3-q21 CASR
HHC2 145981 19p13.3  ?
HHC3 600740 19q13[1]  ?

Most cases are associated with loss of function mutations in the CASR gene, which encodes a calcium-sensing receptor,[2] expressed in parathyroid and kidney tissue. The perceived lack of calcium levels by the parathyroid leads to constitutively high levels of parathyroid hormone, and therefore hypercalcemia. Functionally, parathyroid hormone (PTH) (aka parathormone or parathyrin) increases calcium resorption from the bone and increases phosphate excretion from the kidney which increases serum calcium and decreases serum phosphate.

Another form has been associated with chromosome 3q.[3]

Differential diagnosis[edit]

This condition is indicated by the presence of hypercalcemia (elevated levels of calcium in the blood) at the same time with hypocalciuria (low levels of calcium in the urine). (Usually elevated calcium levels in the blood are correlated with elevated calcium urine levels, as a properly sensing kidney works to excrete the mineral.) A family history could reinforce the diagnosis.

Treatment[edit]

No treatment is generally required, as bone demineralisation and kidney stones are relatively uncommon in the condition.[4]

Functions of the Calcium-sensing Receptor[edit]

  • Parathyroid gland: mediates negative feedback mechanisms relating to PTH secretion (PTH secretion should decrease if there is a high blood calcium level). Abnormalities in the CASR here cause hypercalcaemia.
  • Kidneys: mediates negative feedback mechanisms relating to calcium reabsorption from the tubular system (reabsorption should decrease if there is a high blood calcium level). Abnormalities in the CASR here cause both hypercalcaemia and hypocalciuria.

Clinical features[edit]

  • Most cases: Asymptomatic (unlike primary hyperparathyroidism)
  • Hypercalcaemia
  • Hypocalciuria ( Ca excretion rate < 0.02 mmol/L).
  • Normal to high PTH (CaSR mutation causes the parathyroid glands to be less sensitive to serum calcium, resulting in normal to high PTH despite elevated serum calcium)
  • Hypermagnesaemia

References[edit]

  1. ^ Lloyd SE, Pannett AA, Dixon PH, Whyte MP, Thakker RV (January 1999). "Localization of familial benign hypercalcemia, Oklahoma variant (FBHOk), to chromosome 19q13". Am. J. Hum. Genet. 64 (1): 189–95. doi:10.1086/302202. PMC 1377717. PMID 9915958. 
  2. ^ "A Practical Approach to Hypercalcemia - May 1, 2003 - American Family Physician". Retrieved 2009-03-29. 
  3. ^ Chou YH, Brown EM, Levi T, et al. (July 1992). "The gene responsible for familial hypocalciuric hypercalcemia maps to chromosome 3q in four unrelated families". Nat. Genet. 1 (4): 295–300. doi:10.1038/ng0792-295. PMID 1302026. 
  4. ^ "Familial Hypouricemic Hypercalcemia". Retrieved 2009-06-07.