The most common cause of high calcium levels is primary hyperparathyroidism, a condition where one or more of the parathyroid glands produce too much hormone. Together with cancer-related causes, it accounts for roughly 90% of all cases. If your blood test came back with elevated calcium, hyperparathyroidism is the first thing your doctor will investigate.
How Parathyroid Problems Raise Calcium
Your four parathyroid glands, each about the size of a grain of rice, sit behind your thyroid in your neck. Their job is to keep blood calcium in a tight range (normally 9 to 10.5 mg/dL) by releasing parathyroid hormone, or PTH. PTH tells your bones to release calcium, your kidneys to hold onto it, and your gut to absorb more of it from food.
In primary hyperparathyroidism, one or more of these glands becomes overactive and pumps out PTH regardless of how much calcium is already in the blood. The gland essentially loses its off switch. Most cases happen in outpatient settings and are caught incidentally on routine blood work, often before a person has any symptoms at all. Women over 50 are affected most frequently.
Cancer as a Cause
Cancer is the most common cause of high calcium among people who are already hospitalized. The cancers most often responsible are breast, lung, kidney, and squamous cell cancers, along with multiple myeloma. Cancer raises calcium through several routes: tumor cells can produce a protein that mimics parathyroid hormone, cancer that has spread to bone can break down bone tissue and release stored calcium, and some tumors activate vitamin D in ways that increase calcium absorption.
One key difference from hyperparathyroidism is timing. Cancer-related high calcium usually shows up when the disease is already advanced and diagnosed, not as a surprise finding on a routine lab panel. If your calcium is elevated and you’re otherwise feeling well, cancer is far less likely to be the explanation.
Less Common Causes
Several other conditions can push calcium above normal, though they make up a small fraction of cases compared to parathyroid disease and cancer.
- Granulomatous diseases. Sarcoidosis, an inflammatory condition that causes clusters of immune cells in the lungs and other organs, leads to high calcium in roughly 10 to 17% of patients. The inflamed tissue converts vitamin D into its active form, which increases calcium absorption from food.
- Vitamin D excess. Taking very high doses of vitamin D supplements over time can raise calcium by boosting intestinal absorption. Standard supplementation doses rarely cause this, but megadosing without monitoring can.
- Medications. Thiazide diuretics, a common type of blood pressure pill, reduce the amount of calcium your kidneys flush out. The average calcium level in thiazide-related cases is about 10.7 mg/dL, just above the normal ceiling. Lithium, used for bipolar disorder, can also raise calcium by making parathyroid cells less responsive to normal feedback signals, and in some cases it may even stimulate parathyroid gland growth.
- Familial hypocalciuric hypercalcemia (FHH). This inherited condition causes mildly elevated calcium that persists for life. It’s harmless but can be mistaken for hyperparathyroidism. A 24-hour urine calcium test distinguishes the two: calcium in the urine runs low in FHH and normal or high in hyperparathyroidism.
What High Calcium Feels Like
Mildly elevated calcium (10.5 to 11.9 mg/dL) often produces no symptoms at all, which is why many people learn about it from a blood test they weren’t expecting. When symptoms do appear, doctors sometimes summarize them as “stones, bones, abdominal groans, and psychic moans” because the effects touch so many systems at once.
Kidney stones form because excess calcium spills into the urine and crystallizes. Bones weaken over time as calcium is pulled from them into the bloodstream, leading to bone pain or fractures. Digestive complaints like constipation, nausea, and stomach pain are common. And because calcium affects how nerve cells communicate, people often report brain fog, trouble concentrating, fatigue, drowsiness, or depression.
Moderate levels (12.0 to 13.9 mg/dL) tend to make these symptoms more noticeable. Above 14.0 mg/dL is considered a hypercalcemic crisis, a medical emergency that can cause confusion, heart rhythm problems, and loss of consciousness.
How the Cause Is Identified
The single most informative test after finding high calcium is a PTH level. This one result splits nearly all cases into two categories. If PTH is elevated (or inappropriately “normal” despite high calcium), the parathyroid glands are the problem. If PTH is suppressed, meaning the glands have correctly tried to shut off, something else is driving the calcium up, and cancer becomes the leading suspect.
That PTH result guides everything that follows. High PTH typically leads to imaging of the parathyroid glands and, in many cases, surgery to remove the overactive gland. Suppressed PTH prompts a search for malignancy or other non-parathyroid causes, often including cancer markers and additional vitamin D testing.
If FHH is a possibility, a 24-hour urine collection for calcium helps clarify the picture. Because FHH can produce PTH levels that look mildly elevated, it occasionally mimics hyperparathyroidism closely enough to cause confusion. The urine test resolves this: people with FHH excrete very little calcium, while those with true hyperparathyroidism excrete normal or high amounts.
What to Expect With Hyperparathyroidism
Because hyperparathyroidism is by far the most common diagnosis behind an elevated calcium level, it’s worth knowing what the path forward looks like. Many people with mild disease and no symptoms are monitored with periodic blood and bone density tests rather than treated immediately. Surgery becomes the recommendation when calcium is significantly elevated, kidney stones have formed, bones have thinned, or kidney function has declined.
The surgery itself, called a parathyroidectomy, removes the overactive gland or glands. It’s typically a short procedure, often performed through a small incision in the neck, and most people go home the same day. Cure rates exceed 95% when performed by an experienced surgeon. Calcium levels usually begin dropping within hours, and most people notice improvements in energy and mental clarity within weeks.