The most common cause of high blood calcium is an overactive parathyroid gland, followed by cancer. Normal calcium in the blood falls between 8.5 and 10.5 mg/dL, and anything above that range is considered elevated. The causes range from a tiny benign growth on a gland in your neck to medications you might already be taking.
Overactive Parathyroid Glands
Primary hyperparathyroidism is the number one cause of high calcium in people who aren’t hospitalized. You have four parathyroid glands, each about the size of a grain of rice, sitting behind your thyroid in your neck. When one of them develops a benign tumor (called an adenoma), it pumps out too much parathyroid hormone, or PTH. That extra PTH does three things simultaneously: it pulls calcium out of your bones, tells your kidneys to hold onto calcium instead of filtering it out, and triggers your body to produce more active vitamin D, which increases calcium absorption from food.
The condition is far more common in women. In the United States, about 233 per 100,000 women have it compared to 85 per 100,000 men. The highest rates are in women aged 70 to 79, where prevalence climbs above 1,000 per 100,000. In Scandinavian studies, 2% to 5% of postmenopausal women are affected. Over time, the constant calcium drain from bone leads to bone loss, fractures, and kidney stones.
Cancer
High calcium is one of the most common metabolic complications of cancer, affecting up to 44% of cancer patients at some point during their illness. In about 80% of these cases, the tumor produces a protein called PTHrP that mimics parathyroid hormone. This protein activates the same bone-breakdown pathway as real PTH, releasing stored calcium into the bloodstream while also telling the kidneys to reabsorb more calcium. Cancers of the lung, breast, kidney, and certain blood cancers like multiple myeloma are the most frequent culprits.
Some cancers raise calcium through a different route: tumors that have spread to bone physically destroy bone tissue and release its calcium directly. In either case, cancer-related high calcium tends to develop faster and reach higher levels than parathyroid-related high calcium.
Medications That Raise Calcium
Several common medications can push your calcium levels up. Thiazide diuretics, a widely prescribed class of blood pressure pills, reduce the amount of calcium your kidneys excrete. They do this by altering how sodium and calcium are exchanged in the kidney’s filtering tubes, essentially swapping more sodium into the urine and pulling more calcium back into the blood.
Lithium, used for bipolar disorder, can also raise calcium by shifting the set point at which your parathyroid glands respond to calcium levels, causing them to tolerate and maintain higher blood calcium than normal. Notably, thiazide diuretics and lithium should not be taken together because thiazides reduce lithium clearance by the kidneys, increasing the risk of lithium toxicity.
Taking too much calcium carbonate (found in antacids and supplements) is another increasingly recognized cause. A condition once called milk-alkali syndrome, now sometimes called calcium-alkali syndrome, can develop when people consume 4 to 10 or more grams of elemental calcium per day. This is well above the recommended daily intake and typically happens when people take large amounts of calcium supplements alongside antacids.
Granulomatous Diseases
Sarcoidosis, tuberculosis, and certain fungal infections cause clusters of immune cells called granulomas to form in the lungs, lymph nodes, or other organs. These granulomas contain activated immune cells (macrophages) that produce large amounts of active vitamin D on their own, completely outside the kidney’s normal regulatory system. Unlike the kidney’s tightly controlled vitamin D production, this process isn’t governed by calcium levels at all. It responds to immune signals instead, so it can run unchecked and flood the body with active vitamin D, which drives up calcium absorption from food.
Research has confirmed that both lung macrophages and lymph node tissue from sarcoidosis patients actively convert vitamin D into its most potent form. This is why people with sarcoidosis are often told to limit vitamin D supplements and sun exposure.
A Genetic Cause That Mimics Parathyroid Disease
Familial hypocalciuric hypercalcemia (FHH) is an inherited condition caused by a mutation in the gene for the calcium-sensing receptor. This receptor acts like a thermostat for calcium in your blood. When the receptor doesn’t work properly, your body’s “thermostat” is set too high, so the parathyroid glands and kidneys behave as though calcium is normal even when it’s elevated.
FHH is important mainly because it can be mistaken for primary hyperparathyroidism, which sometimes leads to unnecessary parathyroid surgery. The key difference shows up in a urine test: people with FHH excrete very little calcium in their urine. A calcium-to-creatinine clearance ratio below 0.01 points to FHH in 80% to 90% of cases. FHH rarely causes symptoms and typically doesn’t need treatment.
How High Calcium Feels
Mild elevations (10.5 to 11.9 mg/dL) often produce no symptoms at all, or only vague ones like fatigue, mild constipation, or difficulty concentrating. Many people discover it incidentally on a routine blood test. As levels climb into the moderate range (12.0 to 13.9 mg/dL), symptoms become more noticeable: increased thirst, frequent urination, nausea, loss of appetite, and muscle weakness.
A hypercalcemic crisis, generally defined as levels above 14.0 mg/dL (or above 3.5 mmol/L), is a medical emergency. At this level, the heart’s electrical rhythm can become unstable, and confusion or even coma can develop. Severe high calcium also damages the kidneys, can trigger pancreatitis, and causes dangerous dehydration because the kidneys lose the ability to concentrate urine.
The classic memory device doctors use for hypercalcemia symptoms is “stones, bones, groans, and moans”: kidney stones, bone pain, abdominal complaints, and mood or cognitive changes.
Why Your Lab Results Might Be Misleading
About half the calcium in your blood is bound to a protein called albumin. Standard blood tests measure total calcium, which includes both the bound and the active (free) forms. If your albumin level is low, which is common in people who are malnourished, hospitalized, or have liver disease, your total calcium reading may look normal even though your free calcium is actually high. The reverse is also true: low albumin can make calcium appear low when it’s fine.
Doctors sometimes use a corrected calcium formula to account for this: corrected calcium equals 0.8 times the difference between normal albumin (usually 4.0) and the patient’s albumin, added to the measured calcium. However, this formula has known limitations and can itself overestimate calcium in people with low albumin, potentially creating a false-positive high reading. When accuracy matters, a direct measurement of ionized (free) calcium is more reliable than any correction formula.
How the Cause Is Identified
Once high calcium shows up on a blood test, the next step is almost always measuring PTH. This single test splits the possible causes into two clear categories. If PTH is high or inappropriately normal despite elevated calcium, the parathyroid glands are likely the problem. If PTH is appropriately suppressed (low), something else is driving calcium up, and the investigation turns toward cancer, vitamin D levels, medications, and granulomatous diseases.
A 24-hour urine calcium collection helps distinguish primary hyperparathyroidism from FHH. Vitamin D metabolite levels (both the storage form and the active form) can reveal whether excess vitamin D from supplements or granulomatous disease is the cause. In cases where cancer is suspected, imaging and a PTHrP blood test help confirm it.