Hypercalcemia, an abnormally high level of calcium in the blood, is overwhelmingly caused by two conditions: overactive parathyroid glands and cancer. Together these account for the vast majority of cases. But the full list of triggers is surprisingly long, stretching from prescription drugs and vitamin supplements to genetic mutations, inflammatory diseases, and even prolonged bed rest. Understanding which cause is at work matters because the treatments differ dramatically.
How Calcium Normally Stays in Check
Your body keeps blood calcium in a narrow range using a feedback loop involving three main players: parathyroid hormone (PTH), vitamin D (in its active form, calcitriol), and calcitonin. When calcium dips, PTH is released quickly to pull calcium from bone and reduce how much the kidneys excrete. Calcitriol works more slowly, boosting absorption of dietary calcium from the gut. When calcium is abundant, PTH secretion drops, and calcitonin steps in to slow the breakdown of bone, letting calcium settle back into storage.1PubMed Central. A calcium homeostasis model: orchestration of fast acting PTH and calcitonin with slow calcitriol Almost every cause of hypercalcemia works by hijacking one or more steps in this system.
Primary Hyperparathyroidism
The single most common cause of hypercalcemia in the general outpatient population is primary hyperparathyroidism. One or more of the four small parathyroid glands behind the thyroid become overactive and pump out excess PTH regardless of how much calcium is already circulating.2PubMed Central. Primary hyperparathyroidism In most cases the culprit is a single benign tumor called an adenoma. Less often, all four glands enlarge (a condition called hyperplasia), and very rarely the cause is parathyroid cancer.
Because PTH drives both bone breakdown and kidney calcium reabsorption, chronically elevated PTH can cause kidney stones, bone thinning, fatigue, and concentration problems. Many people, though, are picked up incidentally on routine blood work when they feel fine. Mild cases are sometimes monitored without surgery, but removing the abnormal gland cures the condition in the overwhelming majority of patients.
Cancer and Hypercalcemia
Among hospitalized patients, cancer overtakes parathyroid disease as the leading cause of high calcium. The mechanisms through which tumors raise calcium are varied, but they broadly fall into a few categories.
Humoral Hypercalcemia of Malignancy
The most common cancer-related mechanism involves tumors secreting a protein called parathyroid hormone-related peptide (PTHrP). This molecule mimics the action of PTH, fooling the body into releasing calcium from bone and retaining it in the kidneys. PTHrP secretion has been detected in a wide range of solid tumors, including lung cancers and renal cell carcinoma.3PubMed Central. Hypercalcemia of malignancy caused by parathyroid hormone-related peptide-secreting pancreatic neuroendocrine tumors (PTHrP-PNETs): Case Report Less common tumors like intrahepatic cholangiocarcinoma can also be the source.4PubMed Central. Humoral Hypercalcemia of Malignancy with a Parathyroid Hormone-Related Peptide-Secreting Intrahepatic Cholangiocarcinoma Accompanied by a Gastric Cancer This mechanism, referred to as humoral hypercalcemia of malignancy, does not require the cancer to be in or near bone at all; the protein travels through the bloodstream.
Local Bone Destruction
When tumors grow directly in bone, whether as a primary bone cancer or as metastases from cancers that originated elsewhere (breast, prostate, lung), they can stimulate bone-destroying cells at the site. The resulting flood of calcium into the bloodstream is sometimes called local osteolytic hypercalcemia. Multiple myeloma is a classic example: this cancer of plasma cells drives excessive osteoclast-mediated bone destruction, and hypercalcemia is its most frequent metabolic complication.5PubMed Central. Multiple myeloma/hypercalcemia
Other Tumor-Related Mechanisms
Blood cancers can raise calcium through additional routes, including the production of inflammatory cytokines that accelerate bone breakdown or direct leukemic infiltration of bone.6PubMed Central. Hypercalcemia and multiple osteolytic lesions in an adult patient with relapsed pre-B acute lymphoblastic leukemia: a case report Some lymphomas take a different route entirely: the tumor cells produce an enzyme (1-alpha-hydroxylase) that converts vitamin D into its active form, calcitriol, causing unregulated calcium absorption from the gut and bone. This has been documented in diffuse large B-cell lymphoma, where biopsy tissue stained positive for the enzyme.7PubMed Central. Hypercalcemia Associated with the Ectopic Expression of 25-hydroxyvitamin D3-1α-hydroxylase in Diffuse Large B-cell Lymphoma
Worth noting: hypercalcemia in cancer patients is generally a sign of advanced disease and tends to carry a poor prognosis. Recognizing and treating the elevated calcium is important for comfort and function, but the underlying cancer drives the long-term outlook.
Granulomatous Diseases
Sarcoidosis, tuberculosis, and certain fungal infections share a feature with some lymphomas: the immune cells involved (activated macrophages) produce excess calcitriol outside the normal regulatory loop. In sarcoidosis the mechanism is well established. The granulomas themselves convert vitamin D into its active form, leading to increased calcium absorption from the gut.8PubMed Central. Sarcoidosis and calcium homeostasis disturbances-Do we know where we stand? Because this process is independent of PTH, it does not respond to the body’s usual feedback controls.
Hypercalcemia in sarcoidosis is not universal. Many patients have the disease without ever developing high calcium. But when it does occur, it can be the presenting symptom that leads to the sarcoidosis diagnosis in the first place. Treatment typically involves corticosteroids, which suppress the overactive macrophages and bring calcitriol levels back down. Avoiding excessive sunlight exposure and dietary vitamin D may also help, since either can feed the overproduction.
Medications and Supplements
A number of drugs and supplements can push calcium levels above normal, sometimes in unexpected ways.
Lithium
Lithium, widely used for bipolar disorder, is a well-recognized trigger. It interferes with the calcium-sensing receptor on parathyroid cells, essentially resetting the threshold at which the glands “decide” calcium is high enough. The glands then behave as if calcium is low even when it is not, continuing to secrete PTH. Over time, lithium can also promote enlargement of the parathyroid glands or the growth of adenomas, compounding the problem.9PubMed Central. Lithium-Associated Hypercalcemia: Pathophysiology, Prevalence, Management Managing this is tricky because discontinuing lithium may not be an option for every patient, and the hypercalcemia sometimes persists even after the drug is stopped if gland enlargement has already occurred.
Thiazide Diuretics
Thiazide-type diuretics, commonly prescribed for high blood pressure, reduce how much calcium the kidneys excrete. In most people this shift is mild and clinically meaningless, but in someone who already has subtle hyperparathyroidism or another predisposing condition, a thiazide can tip calcium levels into the abnormal range. Unlike lithium, stopping the diuretic usually corrects the problem.10PubMed Central. Lithium-Associated Hypercalcemia: Pathophysiology, Prevalence, Management
Vitamin D Excess
Taking too much supplemental vitamin D can cause hypercalcemia, though it generally requires quite high doses sustained over weeks or months. The mechanism involves the buildup of 25-hydroxyvitamin D, the storage form. At high enough concentrations, this metabolite can directly activate vitamin D receptors itself, even though it binds less tightly than the fully active form.11PubMed Central. Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment Because vitamin D is fat-soluble and stored in body tissues, toxicity can take a long time to resolve even after supplementation is stopped. This is a growing concern as high-dose vitamin D supplements have become popular and widely available without prescription.
Calcium Supplements and the Milk-Alkali Syndrome
This one surprises people. Taking large amounts of calcium carbonate, often in combination with absorbable alkali, can cause a triad of high calcium, kidney impairment, and metabolic alkalosis. Historically called milk-alkali syndrome (because it was first seen in ulcer patients drinking large quantities of milk with antacids), the condition has made a comeback. The modern version tends to appear in older women taking calcium carbonate for bone health.12PubMed Central. Hypercalcemia and alkalosis due to the milk-alkali syndrome: a case report and review Once considered rare and historical, milk-alkali syndrome is now regarded as the third most common cause of hypercalcemia after hyperparathyroidism and cancer.13PubMed Central. A hidden history of heartburn: The milk-alkali syndrome
The concerning part is that some patients who developed this syndrome were taking doses of calcium that would be considered acceptable, in the range of one to two grams of elemental calcium per day.14PubMed. Calcium carbonate toxicity: the updated milk-alkali syndrome; report of 3 cases and review of the literature Pre-existing mild kidney impairment, dehydration, or concurrent use of thiazide diuretics can lower the threshold for trouble.
Genetic Causes
Some people are born with mutations that alter how the body senses or handles calcium, leading to lifelong hypercalcemia.
The best-known example is familial hypocalciuric hypercalcemia (FHH). This is caused by an inherited loss-of-function mutation in the calcium-sensing receptor gene. With one copy of the mutated gene, the body’s calcium “thermostat” is set slightly too high, so the parathyroid glands tolerate higher calcium levels before dialing back PTH. The result is mild, lifelong hypercalcemia that is typically harmless and requires no treatment.15PubMed. Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia The main clinical importance of FHH is distinguishing it from primary hyperparathyroidism, because unnecessary parathyroid surgery in FHH patients will not fix the calcium level and can cause harm.
A different genetic condition, caused by mutations in the CYP24A1 gene, leads to a loss of the enzyme responsible for breaking down active vitamin D. Without this enzyme, even normal amounts of vitamin D (including standard infant prophylaxis doses) can cause severe hypercalcemia. The condition has been identified in infants who developed dangerously high calcium after routine vitamin D supplementation.16PubMed. Mutations in CYP24A1 and Idiopathic Infantile Hypercalcemia Adults carrying these mutations may go undiagnosed until they are exposed to higher vitamin D intake or undergo testing for recurrent kidney stones.
Thyroid Disease and Other Endocrine Triggers
Hyperthyroidism can raise calcium levels through a mechanism unrelated to the parathyroid glands. Excess thyroid hormone accelerates bone turnover, and the resulting release of calcium from bone can push blood levels above normal. Mild, asymptomatic calcium elevation has been documented in up to about one in five patients with hyperthyroidism.17PubMed Central. An Unusual Case of Hypercalcemia Associated with Graves’ Disease and Vitamin D Deficiency In most of these patients, the hypercalcemia resolves once thyroid hormone levels are brought under control. When high calcium persists despite treating the thyroid, doctors look for concurrent primary hyperparathyroidism, which occasionally coexists.
Adrenal insufficiency (Addison’s disease) is a rarer endocrine trigger. Cortisol normally suppresses intestinal calcium absorption and promotes its excretion by the kidneys, so when cortisol drops, calcium can rise. Pheochromocytomas, tumors of the adrenal medulla, have also been linked to hypercalcemia in case reports, sometimes through ectopic PTHrP secretion.
Immobilization
Prolonged bed rest or immobilization after a spinal cord injury, stroke, or major fracture can cause significant hypercalcemia, particularly in people with already high bone turnover such as adolescents or patients with Paget’s disease. When mechanical loading on the skeleton stops, bone breakdown outpaces bone formation, and calcium floods the bloodstream.18PubMed Central. Immobilization induced hypercalcemia The diagnosis is easy to overlook because patients are often critically ill or recovering from trauma, and high calcium is attributed to other causes before immobilization is considered. Rehydration and, when possible, early mobilization are the main treatments.
Kidney Disease and Tertiary Hyperparathyroidism
Chronic kidney disease causes a chain of mineral imbalances. Failing kidneys cannot activate vitamin D or excrete phosphate efficiently, so calcium absorption drops and phosphate levels climb. The parathyroid glands respond by ramping up PTH production, a state called secondary hyperparathyroidism. This initially keeps calcium in the normal range, but over years the parathyroid glands can become enlarged and autonomous, secreting PTH even when calcium is already high. This progression is called tertiary hyperparathyroidism.19PubMed Central. Secondary and Tertiary Hyperparathyroidism in Chronic Kidney Disease: An Endocrine and Renal Perspective It typically emerges after long-standing kidney disease and is especially common after kidney transplantation, when the new kidney restores vitamin D activation while the overgrown parathyroid glands continue their excess PTH output.20PubMed. Secondary and tertiary hyperparathyroidism
How Doctors Figure Out the Cause
Given the wide range of triggers, a systematic approach is essential. The first and most informative test is measuring intact PTH alongside calcium. If PTH is elevated or inappropriately normal (meaning it has not suppressed in response to high calcium), the problem is PTH-driven, and primary hyperparathyroidism tops the list. A urine calcium test helps separate primary hyperparathyroidism (where urine calcium is usually normal or high) from familial hypocalciuric hypercalcemia (where it is characteristically low).
If PTH is suppressed, the cause is not coming from the parathyroid glands. Doctors then look at PTHrP levels (elevated in humoral hypercalcemia of malignancy), vitamin D metabolites (calcitriol is high in granulomatous disease and some lymphomas; 25-hydroxyvitamin D is high in supplement toxicity), and serum protein electrophoresis to screen for myeloma.21Oxford Academic (Nephrology Dialysis Transplantation). Hypercalcemia: etiology and management A thorough medication and supplement history can also point to drug-induced causes that might otherwise be missed.
Cancer-Associated Hypercalcemia in Animals
Hypercalcemia from cancer is not unique to humans. The same PTHrP-driven mechanism is the most common cause of cancer-related high calcium in dogs, and it is recognized across other domestic species as well.22PubMed Central. Animal Models of Cancer-Associated Hypercalcemia Lymphoma in dogs is a particularly frequent culprit, and veterinarians routinely check calcium as part of the workup. The overlap is more than a curiosity: animal models of cancer-associated hypercalcemia have helped researchers understand PTHrP biology and test treatments that eventually translated back to human medicine. Cats, horses, and ferrets have also been documented with PTHrP-secreting tumors, though the condition is less common in those species than in dogs.