What causes a high blood calcium level?

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A mild elevation of blood calcium usually doesn’t need immediate treatment.

Q: I recently had routine blood work done, and my calcium level was above the normal range at 10.7 mg/dL. Is it dangerous?

A: Your first step should be to get a second blood test to be sure that the first results were not due to a laboratory error. If your repeat test again shows a calcium level slightly above normal, this would confirm mild hypercalcemia.

Mild hypercalcemia is usually defined as a blood calcium level above 10.4 mg/dL (the upper limit of normal) but less than 12.0 mg/dL. Most people with calcium levels in this range don’t have any symptoms specifically caused by the hypercalcemia.

A mild elevation of blood calcium usually doesn’t need immediate treatment. However, it is important to find the cause of your hypercalcemia.

The most common cause of mild hypercalcemia is an overproduction of parathyroid hormone (a condition called hyperparathyroidism). Parathyroid hormone is made by four small parathyroid glands that rest on the back side of the thyroid gland in the neck. Parathyroid hormone acts on the bones to release calcium into the blood.

The parathyroid glands normally produce enough parathyroid hormone to keep blood-calcium levels in the normal range. If blood calcium gets above 10.4 mg/dL, the glands should turn off parathyroid production.

Hypercalcemia caused by hyperparathyroidism happens when:

• All four glands keep putting out more parathyroid hormone despite a high blood-calcium level.

• One of the glands develops a benign growth called a parathyroid adenoma. The adenoma functions independently and does not turn off when calcium rises above normal.

• Parathyroid cancer is present — though this is very rare. Less than 100 cases per year are reported in the United States.

Another reason for hypercalcemia is excessive intake of vitamin D, especially when combined with supplemental calcium. Unless instructed by a doctor, people should not take more than 4,000 IU of vitamin D daily.

Some medications can raise calcium levels. Examples include hydrochlorothiazide, chlorthalidone and lithium.

It sounds as though you are a bit surprised by the blood test result showing hypercalcemia. But this is not unusual. Mild hypercalcemia is often discovered when a person has had a routine blood chemistry panel done — often for reasons unrelated to calcium concerns. A calcium level is a standard part of full chemistry panels.

Your doctor will likely ask you about vitamin D and calcium intake, review your medications, and will order a blood parathyroid hormone test. Additional testing is needed when a person has other symptoms or a very high calcium level, especially if the parathyroid hormone blood test is normal.

(Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)