Familial hypocalciuric hypercalcemia type 3

Other Names: HHC3; Familial benign hypercalcemia, type 3; FBH3; Hypercalcemia, familial benign, type 3; Hypercalcemia, familial benign, Oklahoma type; Familial benign hypercalcemia, Oklahoma variant; FBHOk; Hypocalciuric hypercalcemia, familial, type 3

Familial hypocalciuric hypercalcemia (FHH) type 3 is one of three recognized types of FHH, an inherited condition that causes abnormally high levels of calcium in the blood (hypercalcemia). FHH also causes high levels of parathyroid hormone (PTH) and low levels of calcium in the urine (hypocalciuria).

Cause

FHH type 3 is caused by a mutation in the AP2S1 gene.

Inheritance

Autosomal dominant pattern, a 50/50 chance.

It is inherited in an autosomal dominant maaner.

Signs and symptoms

In most cases, FHH does not cause signs or symptoms. However, some people with FHH do report symptoms. Signs and symptoms that may occur in some people with FHH type 3 include high magnesium in addition to calcium, general symptoms of hypercalcemia (such as weakness, fatigue, muscle pain, constipation, depression, confusion, and excessive thirst), lower bone mineral density, behavioral disorders, and learning disabilities.

Diagnosis

As most cases of FHH are asymptomatic and benign, the diagnosis of FHH is less likely to be made. Typically, diagnosis is made in the pursuit of uncovering the etiology of hypercalcemia. Calcium levels are often in the high normal range or slightly elevated. Commonly, the parathyroid hormone level is checked and may be slightly elevated or also on the high normal end. Normally, high calcium should cause low PTH and so this level of PTH is inappropriately high due to the decreased sensitivity of the parathyroid to calcium. This may be mistaken for primary hyperparathyroidism.

However, evaluation of urine calcium level will reveal a low level of urine calcium. This too is inappropriate as high serum calcium should result in high urine calcium. If urine calcium is not checked, this may lead to parathyroidectomy for presumed primary hyperparathyroidism.

Additionally as the name implies, there may be a family history of benign hypercalcemia.

Ultimately, diagnosis of familial hypocalciuric hypercalcemia is made — as the name implies — by the combination of low urine calcium and high serum calcium.

Treatment

Treatment is generally not necessary in people with no signs or symptoms of FHH. In severe cases however, removal of the parathyroid gland (parathyroidectomy) may be recommended.

NIH genetic and rare disease info

• NIH info on Familial hypocalciuric hypercalcemia type 3

Familial hypocalciuric hypercalcemia type 3 is a rare disease.