Familial hypocalciuric hypercalcemia (FHH) is a rare autosomal dominant condition. It occurs as a result of mutations in the calcium-sensing receptor gene (CASR) causing decreased receptor activity. Patients have mild hypercalcemia, hypocalciuria, hypermagnesemia, hypophosphatemia. Parathyroid hormone is normal or mildly elevated.
In most of the cases, familial hypocalciuric hypercalcemia (FHH1) results from loss-of-function mutations in the calcium-sensing receptor gene on the long arm of chromosome 3 (over 85%). FHH is usually a benign condition in patients who have the heterozygous mutation. The patient presents with the milder disorder and incidentally has a mild elevation in calcium and normal or mildly elevated PTH. The patients with a homozygous mutation can have severe hypercalcemia with marked hyperparathyroidism, fractures, and failure to thrive. Other rare cases of familial hypocalciuric hypercalcemia, FHH2 and FHH3 are linked to a mutation on chromosome 19. FHH can rarely be caused by autoantibodies directed against the calcium-sensing receptor leading to decreased calcium-sensing receptor (CaSR) activity. This type of FHH should be considered in case of the strong family history of autoimmune disorders.
FHH is a rare condition inherited in an autosomal dominant pattern equally distributed between the sexes. Its true prevalence is not known. It has been estimated to be in the range of 1 in 78 000 compared with that of primary hyperparathyroidism of 1 in 1000, but the true prevalence is likely to be higher, due to its subclinical nature in many cases.
The calcium-sensing receptor (CaSR) is expressed in multiple tissues, including the parathyroid glands and kidneys. In the parathyroid gland, activation of the CaSR suppresses PTH secretion, stimulates calcitonin secretion and enhances renal calcium excretion. Studies have shown expression of the CaSR in all segments of the nephron, with the greatest expression in the cortical thick ascending limb of Henle's loop.
The CaSR senses small changes in the serum ionized calcium concentration. The loss of function mutations in the (CaSR) gene in parathyroid gland increases the set point for calcium sensing. It makes the parathyroid glands less sensitive to calcium, and a higher than normal serum calcium level is required to reduce PTH release. In the kidney, this defect leads to an increase in tubular calcium and magnesium reabsorption resulting in hypercalcemia, hypocalciuria, and frequently high normal levels of serum magnesium.
FHH patients are usually asymptomatic or have few symptoms associated with hypercalcemia, probably due to its very mild nature. The usual symptoms are fatigue, weakness, constipation, polyuria, polydipsia, renal insufficiency, or a headache. Other symptoms include chondrocalcinosis or mental problems. Occasionally, patients have pancreatitis. An abnormally functioning CaSR might cause intraductal calcification and increased risk of pancreatitis. There is no increased incidence of fractures in FHH. FHH does not protect against the age-related bone loss.
The diagnosis of FHH can be easy to make in an asymptomatic hypercalcemic patient with a family history of hypercalcemia, a personal or family history of failed neck exploration, or normal serum PTH. More than 99% of the filtered calcium gets reabsorbed despite the presence of hypercalcemia. The 24 hours urinary calcium excretion is less than 100 mg/24 hours. The Ca/Cr excretion ratio is very low. It is calculated as follows: [UCa × SCr] / [SCa × UCr], where UCa is the urinary calcium concentration, SCr is the serum creatinine, SCa is the serum calcium concentration, and UCr is the urinary creatinine concentration, all in mg/dl. The Ca/Cr clearance ratio is less than 0.01 in 80% of cases. All patients with calcium/creatinine clearance ratio of 0.020 or less should be tested for mutations in the CaSR gene. Serum Magnesium is in the upper-normal range or mildly elevated; whereas, serum magnesium tends to be normal or low in primary hyperparathyroidism.
The differentiation between FHH and primary hyperparathyroidism is more difficult in the absence of a family history of hypercalcemia if PTH levels are normal and if the Ca/Cr clearance ratio is greater than 0.01 and less than 0.02. The age at diagnosis of hypercalcemia and family history are important. Detection of asymptomatic hypercalcemia before the age of 40 years or so favors the diagnosis of FHH. Obtaining serum calcium values from first-degree relatives in the absence of a family history can be helpful.
Other causes of PTH-dependent hypocalciuric hypercalcemia should be ruled out, for example, vitamin D deficiency, very low calcium intake, mild renal insufficiency, and treatment with thiazide diuretics or lithium. Correction of any of these abnormalities will lead to hypercalciuria if the patient has PHPT. Serum levels of fibroblast growth factor 23 (FGF-23) may be elevated in patients with PHPT.
Calcium infusion has also been used to distinguish renal calcium handling in FHH from that in primary hyperparathyroidism (PHPT). In PHPT, a rising filtered load of calcium increases urinary calcium excretion, whereas this response is absent in FHH.
FHH is usually a benign disorder and patients with FHH, for the most part, do not develop complications from their disorder. The calcium/PTH levels are usually stable over the years. Subtotal parathyroidectomy does not cure the disorder. Educating and reassuring the patient and affected family members about the benign nature of this condition is very important. This communication avoids unnecessary and expensive monitoring and unnecessary parathyroid exploration in the patient and relatives. Rarely, in a patient with atypical features, such as pancreatitis, total parathyroidectomy may be indicated to reduce the risk of further attacks of pancreatitis. The CaSR represents a potentially important therapeutic target for disorders in which the receptor is hypoactive, such as FHH. Calcimimetics and calcilytics (CaSR antagonists) can play a pharmacological role in improving defective calcium sensing in inherited or acquired disorders of the CaSR.
It is important to distinguish asymptomatic primary hyperparathyroidism from FHH because FHH is a benign inherited condition that typically does not require parathyroidectomy, nor will it be routinely cured by it. Although it is not difficult to differentiate patients with typical biochemical findings of either FHH or primary hyperparathyroidism, it can be challenging to differentiate patients with atypical presentations of either disease and in the absence of family history. Family screening and education are mandatory to avoid unnecessary surgery in the hypercalcemic family members.