The most common causes of low blood calcium are vitamin D deficiency, surgery on the thyroid or parathyroid glands, and chronic kidney disease. Normal total calcium in the blood ranges from 8.8 to 10.4 mg/dL, and levels below 8.8 mg/dL are considered low. Which cause tops the list depends on the setting: in hospitals, post-surgical damage to the parathyroid glands is the leading trigger, while in the general population, vitamin D deficiency and kidney disease account for the bulk of cases.
Vitamin D Deficiency and Calcium Absorption
Vitamin D is the gatekeeper for calcium absorption in your gut. Without enough of it, your intestines simply can’t pull adequate calcium from the food you eat, no matter how much dairy or leafy greens are on your plate. When blood calcium starts to dip, your body tries to compensate by ramping up parathyroid hormone (PTH) production. PTH pulls calcium from your bones to keep blood levels stable, which works temporarily but weakens your skeleton over time. This compensatory cycle, called secondary hyperparathyroidism, is extremely common in people with prolonged vitamin D deficiency.
Vitamin D deficiency is widespread. Limited sun exposure, darker skin, obesity, aging, and living at higher latitudes all increase the risk. Because the deficiency develops gradually and the body compensates for a long time, many people walk around with borderline or low calcium without obvious symptoms until the deficit becomes severe.
Post-Surgical Hypoparathyroidism
In clinical settings, the single most frequent cause of hypoparathyroidism, the condition where the parathyroid glands fail to produce enough PTH, is surgery. About 75% of all hypoparathyroidism cases stem from thyroidectomy, parathyroidectomy, or radical neck surgery. The four tiny parathyroid glands sit directly behind the thyroid, and they can be accidentally damaged or removed during these procedures.
Hypocalcemia after thyroid surgery is remarkably common. In one study, it appeared in over 76% of patients post-operatively. The good news is that most cases are temporary: transient hypoparathyroidism occurs in roughly 25 to 83% of neck surgery patients, but permanent hypoparathyroidism develops in only 0.12 to 4.6%. Still, post-thyroidectomy hypocalcemia is the most common complication of the procedure, and severe cases can prolong hospital stays and significantly affect quality of life.
Chronic Kidney Disease
Your kidneys do more than filter waste. They activate vitamin D into its usable form, and they clear excess phosphorus from your blood. When the kidneys fail, both functions break down simultaneously. Phosphorus builds up and binds to calcium, pulling it out of circulation and depositing it into bone and soft tissues. At the same time, the kidneys can no longer convert vitamin D into the active form your gut needs to absorb calcium. This double hit makes low calcium nearly universal in advanced kidney disease.
Magnesium: The Overlooked Cofactor
Low magnesium is one of the most underappreciated causes of low calcium. Your parathyroid glands need magnesium to produce and release PTH. When magnesium drops too low, PTH secretion stalls, and calcium falls as a direct consequence. This is called secondary hypocalcemia, and it has a distinctive clinical feature: it won’t respond to calcium supplementation alone. Until magnesium levels are corrected, calcium levels stay stubbornly low. Magnesium deficiency commonly results from heavy alcohol use, prolonged diarrhea, certain diuretics, and poor dietary intake.
Medications That Lower Calcium
Several widely prescribed drugs can drive calcium levels down. Denosumab (sold as Prolia), used for osteoporosis, works by blocking a protein that activates bone-dissolving cells. As a side effect, it can significantly lower blood calcium. The FDA added a boxed warning to denosumab after data showed it carried a higher risk of severe hypocalcemia compared to bisphosphonates, another class of osteoporosis drugs. The risk is especially pronounced in people with advanced chronic kidney disease, whose calcium regulation is already compromised.
Other drug categories that can lower calcium include certain anti-seizure medications, proton pump inhibitors used for acid reflux, and some chemotherapy agents. If you’re on long-term medication and experience symptoms of low calcium, the drug itself may be a contributing factor worth discussing with your provider.
How Low Calcium Feels
Mild hypocalcemia often produces no symptoms at all. As levels drop further, the earliest signs tend to involve tingling or numbness around the lips, fingertips, and toes. Muscle cramps, stiffness, and spasms follow, particularly in the hands and feet. In more severe cases, you may notice twitching in the facial muscles or a sensation that your hand is locking into a clenched position.
These neuromuscular symptoms happen because calcium plays a central role in how your nerves fire. When calcium is too low, nerves become hyperexcitable, meaning they fire more easily and sometimes spontaneously. Severe, untreated hypocalcemia can progress to seizures, abnormal heart rhythms, and difficulty breathing due to spasm of the airway muscles.
Why Albumin Levels Matter
About half the calcium in your blood is bound to a protein called albumin. When albumin is low, which is common in people who are malnourished, have liver disease, or are critically ill, total calcium readings on a blood test will appear falsely low. The actual amount of free, active calcium (ionized calcium) may be perfectly normal.
To account for this, labs and clinicians use a correction formula that adjusts the total calcium reading based on your albumin level. If your total calcium comes back low, it’s worth knowing whether your albumin was also low, because the “true” calcium may not be as concerning as the number suggests. Ionized calcium, measured between 4.7 and 5.2 mg/dL normally, gives a more accurate picture when albumin is abnormal.
Rare Genetic Causes
In a small number of people, low calcium has a genetic basis. Pseudohypoparathyroidism is a group of inherited conditions where the body produces enough PTH but can’t respond to it properly. The problem lies in a signaling protein that normally carries the PTH message inside cells. When this protein is defective, due to mutations in a gene called GNAS, PTH essentially knocks on the door but nobody answers. The result is low calcium and high phosphorus despite normal or even elevated PTH levels.
The inheritance pattern is unusual. The condition only causes hormone resistance when the defective gene is inherited from the mother. A father with the same genetic change passes on a different, milder condition without calcium problems. These disorders are rare but important to identify because they require lifelong management and can involve other hormonal resistances and distinctive physical features.