Hypercalcemia

Hypercalcemia is a disorder commonly encountered by primary care physicians. Approximately one in 500 patients who are treated in a general medicine clinic have undiagnosed primary hyperparathyroidism, the leading cause of hypercalcemia. (1-4) The diagnosis of hypercalcemia most often is made incidentally when a high calcium level is detected in blood samples. The principal challenges in the management of hypercalcemia are distinguishing primary hyperparathyroidism from conditions that will not respond to parathyroidectomy and knowing when it is appropriate to refer the patient for surgery. It is essential that physicians know how to evaluate and optimally manage patients with hypercalcemia, because treatment and prognosis vary according to the underlying disorder.

Pathophysiology of Hypercalcemia

The skeleton contains 98 percent of total body calcium; the remaining 2 percent circulates throughout the body. One half of circulating calcium is free (ionized) calcium, the only form that has physiologic effects. The remainder is bound to albumin, globulin, and other inorganic molecules. Low albumin levels can affect the total serum calcium level. Directly measuring the free calcium level is more convenient and accurate, but the following formula can be used to calculate the corrected total serum calcium level:

Corrected calcium = (4.0 g per dL--[plasma albumin]) 3 0.8 + [serum calcium]

Parathyroid hormone (PTH), 1,25-dihydroxyvitamin [D.sub.3] (calcitriol), and calcitonin control calcium homeostasis in the body (Table 1). Increased bone resorption, increased gastrointestinal absorption of calcium, and decreased renal excretion of calcium cause hypercalcemia. Normal serum calcium levels are 8 to 10 mg per dL (2.0 to 2.5 mmol per L, Figure 1), although the exact range can vary among laboratories. Normal ionized calcium levels are 4 to 5.6 mg per dL (1 to 1.4 mmol per L). Hypercalcemia is considered mild if the total serum calcium level is between 10.5 and 12 mg per dL (2.63 and 3 mmol per L). (5) Levels higher than 14 mg per dL (3.5 mmol per L) can be life threatening.

[FIGURE 1 OMITTED]

PTH is an 84-amino acid hormone produced by the four pea-sized parathyroid glands posterior to the thyroid gland. In response to low serum calcium levels, PTH raises calcium levels by accelerating osteoclastic bone resorption and increasing renal tubular resorption of calcium. It also increases calcitriol, which indirectly raises serum calcium levels. PTH causes phosphate loss through the kidneys. Thus, in patients with PTH-mediated hypercalcemia, serum phosphate levels tend to be low.

Vitamin D is a steroid hormone that is obtained through the diet or produced by the action of sunlight on vitamin D precursors in the skin. Calcitriol, the active form of vitamin D, is derived from successive hydroxylation of the precursor cholecalciferol, first in the liver (25-hydroxylation), then in the kidneys (1-hydroxylation). Adequate vitamin D is necessary for bone formation. However, the principal target for vitamin D is the gut, where it increases the absorption of calcium and phosphate. Thus, in vitamin D-mediated hypercalcemia, serum phosphate levels tend to be high.

Calcitonin is a 32-amino acid hormone produced by the parafollicular C cells of the thyroid. Calcitonin is a weak inhibitor of osteoclast activation and opposes the effects of PTH on the kidneys, thereby promoting calcium and phosphate excretion. Calcitonin levels might be elevated in pregnant patients and in patients with medullary carcinoma of the thyroid. However, there are no direct clinical sequelae, and serum calcium levels usually are normal.

PTH-related peptide (PTHrP) is the principal mediator in hypercalcemia associated with solid tumors. (6) PTHrP is homologous with PTH at the amino terminus, the region that comprises the receptor-binding domain. PTHrP binds the PTH receptor and mimics the biologic effects of PTH on bones and the kidneys.

Clinical Manifestations of Hypercalcemia

The optimal concentration of serum ionized calcium is essential for normal cellular function. Hypercalcemia leads to hyperpolarization of cell membranes. Patients with levels of calcium between 10.5 and 12 mg per dL can be asymptomatic. (7) When the serum calcium level rises above this stage, multisystem manifestations become apparent (Table 2). This constellation of symptoms has led to the mnemonic "Stones, bones, abdominal moans, and psychic groans," which is used to recall the signs and symptoms of hypercalcemia, particularly as a result of primary hyperparathyroidism.

Neuromuscular effects include impaired concentration, confusion, corneal calcification, fatigue, and muscle weakness. (8) Nausea, abdominal pain, anorexia, constipation, and, rarely, peptic ulcer disease or pancreatitis are among the gastrointestinal manifestations. The most important renal effects are polydipsia and polyuria resulting from nephrogenic diabetes insipidus, and nephrolithiasis resulting from hypercalciuria. Other renal effects include dehydration and nephrocalcinosis. Cardiovascular effects include hypertension, vascular calcification, and a shortened QT interval on the electrocardiogram. Cardiac arrhythmias are rare. Bone pain can occur in patients with hyperparathyroidism or malignancy. Osteoporosis of cortical bone, such as the wrist, is mainly associated with primary hyperparathyroidism. (9) Excess PTH also can result in subperiosteal resorption, leading to osteitis fibrosa cystica with bone cysts and brown tumors of the long bones.

Differential Diagnosis for Hypercalcemia

Primary hyperparathyroidism and malignancy account for more than 90 percent of hypercalcemia cases. These conditions must be differentiated early to provide the patient with optimal treatment and accurate prognosis. Humoral hypercalcemia of malignancy implies a very limited life expectancy--often only a matter of weeks. On the other hand, primary hyperparathyroidism has a relatively benign course.

The causes of hypercalcemia can be divided into seven categories: hyperparathyroidism, vitamin D-related causes, malignancy, medications, other endocrine disorders, genetic disorders, and miscellaneous causes (Table 3). Evaluation of a patient with hypercalcemia (Figure 2) should include a careful history and physical examination focusing on clinical manifestations of hypercalcemia, risk factors for malignancy, causative medications, and a family history of hypercalcemia-associated conditions (e.g., kidney stones).

[FIGURE 2 OMITTED]

HYPERPARATHYROIDISM

Increased screening of calcium levels and wide availability of reliable assays for intact PTH levels have led to more frequent and earlier diagnoses of primary hyperparathyroidism. In 80 percent of cases, a single parathyroid adenoma is responsible. However, hyperparathyroidism also can result from hyperplasia of the parathyroid glands or, rarely, parathyroid carcinoma. In primary or tertiary hyperparathyroidism, PTH levels are normal or high in the setting of hypercalcemia (Figure 3).

[FIGURE 3 OMITTED]

In many patients, primary hyperparathyroidism progresses very slowly. Patients should be considered for parathyroidectomy only if they meet criteria recommended by the National Institutes of Health Consensus Development Conference (Table 4). (10) [Evidence level C, consensus opinion] The disease will progress in approximately one fourth of patients who do not undergo surgery. (11) Preoperative nuclear imaging of the parathyroids with a sestamibi scan (Figure 4) allows the surgeon to perform unilateral neck dissection, which results in reduced operative time and less morbidity. (12) Risks of parathyroid surgery include permanent hypoparathyroidism and damage to the recurrent laryngeal nerve.

[FIGURE 4 OMITTED]

Chronic renal failure generally causes hypocalcemia. If untreated, prolonged high phosphate and low vitamin D levels can lead to increased PTH secretion and subsequent hypercalcemia. This is termed tertiary hyperparathyroidism.

VITAMIN D-MEDIATED CAUSES

The most commonly available vitamin D supplements consist of 25-hydroxyvitamin [D.sub.2]. In suspected overdose of over- the-counter vitamin D, the level of 25-hydroxyvitamin [D.sub.3] (not 1,25-dihydroxyvitamin [D.sub.3]) should be measured. Macrophages can cause granuloma-forming (i.e., sarcoidosis, tuberculosis, Hodgkin's lymphoma) increased extra-renal conversion of 25-hydroxyvitamin [D.sub.3] to calcitriol. PTH levels are suppressed, and levels of 1,25-dihydroxyvitamin [D.sub.3] are elevated. Hypercalcemia mediated by excessive vitamin D responds to a short course of glucocorticoids if the underlying disease is treated.

HYPERCALCEMIA OF MALIGNANCY

Hypercalcemia of malignancy occurs in several settings. (13) It is mediated most commonly by systemic PTHrP in patients with solid tumors. This is known as the humoral hypercalcemia of malignancy. PTHrP mimics the bone and renal effects of PTH. In contrast to primary hyperparathyroidism, the humoral hypercalcemia of malignancy is associated with suppressed PTH levels and normal calcitriol levels. Extensive bone lysis also can cause malignancy-associated hypercalcemia. Multiple myeloma and metastatic breast cancer can present in this way. In osteolytic hypercalcemia, the alkaline phosphatase level is usually markedly elevated. Hodgkin's lymphoma causes hypercalcemia through increased production of calcitriol.

MEDICATIONS

Thiazide diuretics increase renal calcium resorption and cause mild hypercalcemia that should resolve when the medication is discontinued. Thiazide diuretic therapy can unmask many cases of primary hyperparathyroidism. Consumption of large amounts of calcium carbonate via calcium-containing antacids can lead to hypercalcemia, alkalosis, and renal insufficiency--an uncommon disorder termed milk-alkali syndrome. (14) Lithium use can cause hypercalcemia by increasing the set point of PTH, (15) requiring a higher serum calcium level to switch off PTH secretion. Large doses of vitamin A and its analogs can cause hypercalcemia, which appears to be mediated through increased bone resorption.

OTHER ENDOCRINE DISORDERS

Thyrotoxicosis-induced bone resorption can result in mild hypercalcemia. Volume expansion and glucocorticoid replacement can correct the hypercalcemia that occasionally occurs in patients with adrenal insufficiency. Pheochromocytoma is thought to cause hypercalcemia through the production of PTHrP. Pheochromocytoma may be associated with primary hyperparathyroidism as part of type 1 multiple endocrine neoplasia syndrome.

FAMILIAL HYPOCALCIURIC HYPERCALCEMIA

Familial hypocalciuric hypercalcemia (16) (FHH) is an autosomal-dominant condition with virtually 100 percent penetrance. Most cases are caused by a mutation in the calcium-sensing receptor gene. Patients have moderate hypercalcemia from an early age but relatively low urinary calcium excretion. PTH levels can be normal or only mildly elevated despite the hypercalcemia. This mild elevation can lead to an erroneous diagnosis of primary hyperparathyroidism. The conditions can be differentiated by use of a 24-hour urinary collection for calcium; calcium levels will be high or normal in patients with hyperparathyroidism and low in patients with FHH. Parathyroidectomy is not beneficial in patients with FHH.

MISCELLANEOUS CAUSES

In conditions of high bone turnover, such as Paget's disease and normal growth in children, immobilization can cause hypercalcemia. Hypercalcemia also can occur in the recovery phase of rhabdomyolysis-induced renal injury, when calcium deposited in soft tissue is mobilized.

TREATMENT OF HYPERCALCEMIA

Asymptomatic patients with mild hypercalcemia generally do not benefit from normalization of their serum calcium levels. Patients with calcium levels greater than 14 mg per dL or symptomatic patients with calcium levels greater than 12 mg per dL (Table 5) should be immediately and aggressively treated. (17) The safest and most effective treatment of hypercalcemic crisis is saline rehydration followed by furosemide (Lasix) diuresis, calcitonin, and bisphosphonates.

HYDRATION AND DIURESIS

In patients with mild hypercalcemia, adequate hydration should be encouraged and immobilization discouraged. In symptomatic patients, a loop diuretic (e.g., furosemide) can be prescribed. Recent evidence suggests that estrogen-replacement therapy might be beneficial in postmenopausal women with primary hyperparathyroidism. (18) [Evidence level B, lower-quality randomized controlled trial]

In patients with severe hypercalcemia, the mainstay of management is aggressive intravenous rehydration. Normal saline should be used to achieve a urine output of 200 mL per hour. Only when the intravascular volume has been restored should a loop diuretic be used in low dosages (e.g., furosemide, 10 to 20 mg) to further lower the serum calcium level if necessary.

PHARMACOLOGIC AGENTS

In malignancy-associated hypercalcemia, intravenous pamidronate (Aredia), 60 to 90 mg, can be given by four-hour infusion. (13) This agent often will normalize the serum calcium level, but peak effects do not occur until 48 to 72 hours after infusion. Caution must be used with bisphosphonates (19) in patients with renal impairment. In severe hypercalcemia refractory to saline diuresis, calcitonin (Calcimar, Miacalcin) can be given every six hours. This treatment has a rapid onset but short duration of effect, and patients develop tolerance to the calcium-lowering effect. Other antiresorptive agents that are used occasionally include plicamycin (Mithracin) and gallium nitrate (Ganite). In hypercalcemia mediated by vitamin D and in hematologic malignancies (e.g., myeloma, lymphoma), glucocorticoids are the first line of therapy after fluids.

DIALYSIS

In cases of resistant, life-threatening hypercalcemia, hemodialysis against a low-calcium dialysate is more effective than peritoneal dialysis in lowering serum calcium levels. Therapy for the underlying condition should be instituted as soon as possible. Consultation with an endocrinologist is recommended. surgery In cases of hypercalcemic crisis resulting from primary hyperparathyroidism, urgent parathyroidectomy is potentially curative. (20)

The authors thank Carolyn King for assistance in the preparation of this manuscript.

The authors indicate that they do not have any conflicts of interest. Sources of funding: none reported.

REFERENCES

(1.) Silverberg SJ, Fitzpatrick LA, Bilezikian JP. Hyperparathyroidism. In: Becker KL, ed. Principles and practice of endocrinology and metabolism. 2d ed. Philadelphia: Lippincott, 1995:512-9.

(2.) Bringhurst FR, Demay MB, Kronenberg HM. Hormones and disorders of mineral metabolism. In: Wilson JD, ed. Williams Textbook of endocrinology. 9th ed. philadelphia: Saunders, 1998:1155-1209.

(3.) Wermers RA, Khosla S, Atkinson EJ, Hodgson SF, O'Fallon WM, Melton LJ 3d. The rise and fall of primary hyperparathyroidism: a population-based study in Rochester, Minnesota, 1965-1992. Ann Intern Med 1997;126:433-40.

(4.) Al Zahrani A, Levine MA. Primary hyperparathyroidism. Lancet 1997;349:1233-8.

(5.) Bushinsky DA, Monk RD. Electrolyte quintet: calcium. Lancet 1998;352:306-11.

(6.) Strewler GJ. The physiology of parathyroid hormone-related protein. N Engl J Med 2000;342:177-85.

(7.) Shane E. Hypercalcemia: pathogenesis, clinical manifestations, differential diagnosis, and management. In: Favus MJ, ed. Primer on the metabolic bone diseases and disorders of mineral metabolism. 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 1999:183-7.

(8.) Solomon BL, Schaaf M, Smallridge RC. Psychologic symptoms before and after parathyroid surgery. Am J Med 1994;96:101-6.

(9.) Abdelhadi M, Nordenstrom J. Bone mineral recovery after parathyroidectomy in patients with primary and renal hyperparathyroidism. J Clin Endocrinol Metab 1998;83:3845-51.

(10.) NIH conference: diagnosis and management of asymptomatic primary hyperparathyroidism: consensus development conference statement. Ann Intern Med 1991;114:593-7.

(11.) Silverberg SJ, Shane E, Jacobs TP, Siris E, Bilezikian JP. A 10-year prospective study of primary hyperparathyroidism with or without parathyroid surgery. N Engl J Med 1999;341:1249-55.

(12.) Gupta VK, Yeh KA, Burke GJ, Wei JP. 99m-technetium sestamibi localized solitary parathyroid adenoma as an indication for limited unilateral surgical exploration. Am J Surg 1998;176:409-12.

(13.) Mundy GR, Guise TA. Hypercalcemia of malignancy. Am J Med 1997;103:134-45.

(14.) Orwoll ES. The milk-alkali syndrome: current concepts. Ann Intern Med 1982;97:242-8.

(15.) Mallette LE, Eichhorn E. Effects of lithium carbonate on human calcium metabolism. Arch Intern Med 1986;146:770-6.

(16.) Marx SJ. Familial hypocalciuric hypercalcemia. In: Favus MJ ed. Primer on the metabolic bone diseases and disorders of mineral metabolism. 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 1999:195-8.

(17.) Ziegler R. Hypercalcemic crisis. J Am Soc Nephrol 2001;12(suppl 17):S3-9.

(18.) Orr-Walker BJ, Evans MC, Clearwater JM, Horne A, Grey AB, Reid IR. Effects of hormone replacement therapy on bone mineral density in postmenopausal women with primary hyperparathyroidism: four-year follow-up and comparison with healthy postmenopausal women. Arch Intern Med 2000; 160;2161-6.

(19.) Lourwood DL. The pharmacology and therapeutic utility of bisphosphonates. Pharmacotherapy 1998; 18:779-89.

(20.) Fitzpatrick LA, Bilezikian JP. Acute primary hyperparathyroidism. Am J Med 1987;82:275-82.

MARY F. CARROLL, M.D., is director of endocrinology and metabolism at Eastern New Mexico Medical Center, Roswell. She graduated from Trinity College in Dublin, Ireland, and completed a residency in internal medicine and a fellowship in endocrinology and metabolism at the University of New Mexico School of Medicine in Albuquerque.

DAVID S. SCHADE, M.D., is professor of medicine and chief of endocrinology and metabolism at the University of New Mexico School of Medicine and Health Sciences Center. He graduated from Washington University School of Medicine in St. Louis and completed a residency in internal medicine and a fellowship in endocrinology and metabolism at the University of New Mexico School of Medicine.

Address correspondence to David S. Schade, M.D., University of New Mexico Health Sciences Center, Department of Internal Medicine/5-ACC, Division of Endocrinology, 2211 Lomas Blvd. NE, Albuquerque, NM 87131 (dschade@salud.unm.edu). Reprints are not available from the authors.

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