High calcium levels in the blood, a condition called hypercalcemia, are caused by overactive parathyroid glands in the majority of cases. Normal blood calcium falls below 10.5 mg/dL, and anything above that threshold is considered elevated. About 90% of all cases trace back to just two causes: overactive parathyroid glands or cancer. But several other conditions, medications, and even supplements can push calcium levels too high.
Overactive Parathyroid Glands
The single most common cause of high calcium in people who aren’t hospitalized is primary hyperparathyroidism. You have four tiny parathyroid glands in your neck, and their job is to regulate calcium by releasing parathyroid hormone (PTH). When one or more of these glands become overactive, they release too much PTH on their own, ignoring the body’s normal feedback signals. That excess hormone pulls calcium out of your bones, tells your kidneys to hold onto calcium instead of filtering it out, and ramps up calcium absorption from food.
Most people with this condition are diagnosed by accident, when a routine blood test comes back with elevated calcium. It’s especially common in postmenopausal women. Many people have no symptoms at all for years, though over time it can lead to kidney stones, bone thinning, and fatigue.
Cancer-Related Causes
Among hospitalized patients, cancer is the most frequent cause of high calcium. Tumors can raise calcium through several pathways, but the most common one involves a protein that mimics parathyroid hormone. This protein, called PTHrP, shares about 70% of its structure with real PTH, which means it activates the same receptors. It triggers bone breakdown, tells the kidneys to retain calcium, and drives levels up quickly.
Lung cancers (particularly squamous cell carcinoma), kidney cancer, ovarian cancer, and esophageal cancer are among the solid tumors most associated with this mechanism. Certain blood cancers, including adult T-cell leukemia and non-Hodgkin lymphoma, can also produce it. Some of these cancers additionally increase production of the active form of vitamin D, which further amplifies calcium absorption from the gut. Cancer-related hypercalcemia tends to develop faster and reach higher levels than other causes, making it more likely to become a medical emergency.
Medications and Supplements
Several commonly used medications can nudge calcium levels upward. Thiazide diuretics, a class of blood pressure medication, work by changing how your kidneys handle sodium. A side effect of that process is increased calcium reabsorption: your kidneys send calcium back into the bloodstream instead of excreting it in urine. For most people this effect is mild, but in someone who already has borderline high calcium or an undetected parathyroid problem, thiazides can tip the balance. Lithium, used to treat bipolar disorder, can also raise calcium by shifting the set point at which the parathyroid glands respond.
Vitamin D supplements are another increasingly common culprit. Doses above roughly 10,000 IU per day can cause toxicity, pushing blood levels of the vitamin’s storage form above 150 ng/mL and triggering excessive calcium absorption. In the 1940s, doctors prescribed massive doses (200,000 to 300,000 IU daily) for conditions like tuberculosis and rheumatoid arthritis, and hypercalcemia was a predictable result. Today the risk is lower, but people taking high-dose supplements without monitoring can still develop problems.
Calcium supplements themselves carry risk when taken in excess. Daily elemental calcium intake above 2 grams is considered the upper safety limit, and even lower amounts (around 1.7 grams) have triggered a condition called milk-alkali syndrome in people with additional risk factors like dehydration or kidney issues. In one study, more than 20% of heart transplant recipients taking 8 to 12 grams of calcium carbonate daily developed hypercalcemia.
Granulomatous Diseases
Certain inflammatory conditions cause immune cells to produce the active form of vitamin D outside the kidneys, where it’s normally made. Sarcoidosis is the best-known example. In this disease, activated immune cells (macrophages) trapped in the lungs and in clusters of inflamed tissue convert vitamin D into its most potent form, calcitriol. That extra calcitriol supercharges calcium absorption from food and releases calcium from bone, often raising blood levels significantly. Tuberculosis, fungal infections, and other granulomatous diseases can do the same thing through the same pathway.
Prolonged Immobilization
Bones stay strong partly because of the mechanical stress placed on them by movement and weight-bearing. When that stimulus disappears, as during weeks of bed rest after a spinal cord injury, major surgery, or severe illness, the balance between bone building and bone breakdown shifts. Cells that break down bone become more active while cells that build bone slow down. The result is a steady release of calcium into the bloodstream. This form of high calcium is most common in young people (whose bones turn over faster) and in people who already have conditions that accelerate bone loss, including cancer that has spread to bone.
Genetic Causes
A less common but important cause is familial hypocalciuric hypercalcemia (FHH), an inherited condition that affects how the body senses calcium. In the most common form, accounting for about 65% of FHH cases, a mutation in the gene for the calcium-sensing receptor makes both the parathyroid glands and the kidneys less sensitive to calcium. The body essentially perceives high calcium as normal, so it never triggers the correction mechanisms that would bring levels down.
FHH is significant because it looks very similar to primary hyperparathyroidism on blood tests, but it doesn’t require surgery. It should be suspected when calcium remains elevated even after a parathyroid gland has been surgically removed. A key distinguishing feature is that people with FHH excrete very little calcium in their urine, while those with hyperparathyroidism typically excrete more.
How Severity Is Classified
Not all high calcium is equally dangerous. Mild hypercalcemia (10.5 to 11.9 mg/dL) often produces no noticeable symptoms and is frequently found on routine lab work. Many people live with mildly elevated calcium for years. Moderate hypercalcemia (12.0 to 13.9 mg/dL) is more likely to cause symptoms like increased thirst, frequent urination, constipation, and mental fogginess. Levels at or above 14.0 mg/dL are considered a hypercalcemic crisis, a medical emergency that can cause confusion, cardiac rhythm disturbances, and kidney failure.
Symptoms and Complications Over Time
Mild high calcium is often called the “silent” presentation because you may feel perfectly fine. When symptoms do appear, they tend to be vague: fatigue, muscle weakness, nausea, constipation, and difficulty concentrating. Because these overlap with so many other conditions, high calcium is easy to miss without a blood test.
The longer calcium stays elevated, the more damage it can do. Kidney stones are one of the most common complications, forming when excess calcium filtered through the kidneys crystallizes in the urinary tract. Over time, persistent hypercalcemia can also deposit calcium directly in kidney tissue, impairing function. On the cardiovascular side, severely elevated calcium shortens the electrical cycle of the heart, which shows up as a specific change on an EKG and can trigger dangerous arrhythmias, particularly in people taking certain heart medications. Bone health deteriorates too, since many causes of high calcium involve pulling minerals out of the skeleton.
How the Cause Is Identified
When a blood test shows elevated calcium, the first step is measuring parathyroid hormone. This single test splits the diagnostic picture into two broad categories. If PTH is elevated (or inappropriately normal when calcium is high), the problem is PTH-driven, pointing toward hyperparathyroidism or FHH. If PTH is appropriately low, the body is correctly trying to suppress it, and something else is forcing calcium up: cancer, vitamin D excess, granulomatous disease, or medications.
From there, additional tests narrow things down. If PTH is suppressed, measuring PTHrP helps identify cancer-related causes. Vitamin D metabolite levels can reveal toxicity or granulomatous overproduction. Urine calcium measurements help distinguish hyperparathyroidism from FHH. A medication review can identify thiazides, lithium, or excessive supplement use. In most cases, the cause becomes clear relatively quickly, and treatment targets the underlying condition rather than the calcium level alone.