Hypocalcemia (Low Blood Calcium)

What hypocalcemia means

Almost all of the body's calcium is in bone. The small amount in the blood is tightly controlled because nerves, muscles and the heart need it to work properly. Two hormones do most of the control:

• Parathyroid hormone (PTH) from four small glands in the neck raises blood calcium by releasing it from bone, saving it in the kidneys and activating vitamin D.
• Vitamin D (in its active form, calcitriol) increases calcium absorption from food.

Hypocalcemia happens when one of these systems fails, or when calcium is suddenly bound or lost faster than it can be replaced. Low calcium makes nerves overexcitable, which explains the tingling and spasms.

Calcium deficiency from a low dietary intake mainly thins the bones over years. It rarely lowers blood calcium, because the body takes calcium from bone to keep blood levels normal.

Corrected calcium and ionized calcium

About 40% of blood calcium is attached to the protein albumin. If albumin is low, total calcium looks low even when the active, "free" calcium is normal. Labs either report a corrected calcium (adding about 0.8 mg/dL for every 1 g/dL that albumin is below 4 g/dL) or measure ionized calcium directly, which is the most accurate test. Normal ionized calcium is roughly 4.6 to 5.3 mg/dL (1.15 to 1.33 mmol/L).

Symptoms

How you feel depends on how low the calcium is and how fast it fell. A slow, long-term drop can cause few symptoms even at quite low levels.

Acute symptoms

• tingling or numbness around the mouth, in the fingertips and toes
• muscle cramps in the back and legs
• carpopedal spasm: the hands and feet cramp into a bent, pointed position
• twitching of muscles
• anxiety, irritability or confusion
• in severe cases, seizures, throat spasm and heart rhythm problems

These features together are called tetany.

Long-term symptoms

• tiredness, depression and poor memory
• dry, coarse skin, brittle nails and thin hair
• early cataracts
• dental problems in children
• calcium deposits in the brain (basal ganglia) in long-standing hypoparathyroidism

Signs a doctor may check

• Chvostek sign: tapping the face in front of the ear makes the corner of the mouth twitch.
• Trousseau sign: inflating a blood pressure cuff above systolic pressure for 3 minutes triggers a hand spasm. It is the more reliable of the two.
• ECG: low calcium lengthens the QT interval, which raises the risk of dangerous arrhythmias.

Causes of hypocalcemia

Low calcium in newborns

Newborns, especially premature babies, babies of mothers with diabetes and babies who had a difficult birth, can develop low calcium in the first days of life. It may cause jitteriness, poor feeding or seizures and is screened for in at-risk babies.

Hyperventilation can mimic it

Breathing too fast, for example during a panic attack, makes the blood more alkaline. More calcium binds to albumin, so ionized calcium drops temporarily and causes tingling and hand spasms. Total calcium on a blood test is normal.

Diagnosis

Blood tests confirm and explain hypocalcemia:

• total calcium with albumin, or ionized calcium
• parathyroid hormone (PTH)
• magnesium and phosphate
• vitamin D (25-hydroxyvitamin D)
• kidney function
• an ECG to check the QT interval

Low calcium with low or inappropriately normal PTH points to hypoparathyroidism or low magnesium. Low calcium with high PTH points to vitamin D deficiency, kidney disease or PTH resistance.

Treatment

Severe or symptomatic hypocalcemia

This is treated in hospital with calcium gluconate given slowly into a vein, with heart monitoring, followed by an infusion if needed. Magnesium is replaced at the same time, because calcium will not stay up while magnesium is low.

Long-term treatment

• Oral calcium supplements (calcium carbonate or citrate).
• Active vitamin D (calcitriol) for hypoparathyroidism and kidney disease, because these patients cannot activate ordinary vitamin D well.
• Ordinary vitamin D (cholecalciferol) for vitamin D deficiency.
• PTH replacement (palopegteriparatide) for chronic hypoparathyroidism that is hard to control.
• Treating the cause: stopping a causative medicine, treating malabsorption or managing kidney disease.

In hypoparathyroidism, the target is usually the low end of the normal range. Pushing calcium higher increases calcium in the urine and the risk of kidney stones. Regular blood and urine tests guide the dose.

Before starting denosumab or bone infusions

Your doctor should check calcium and vitamin D levels first and correct any deficiency, especially if you have kidney disease, to avoid a sharp drop in calcium.

Frequently asked questions

What are the first signs of low calcium?

The earliest signs are usually tingling or numbness around the mouth and in the fingertips and toes, followed by muscle cramps, twitching and stiffness. Some people feel anxious or tired. Hand and foot spasms, a tight throat or a seizure mean calcium is severely low and need emergency care.

What is the most common cause of hypocalcemia?

In hospital patients, low albumin and critical illness are common causes. Among outpatients, true hypocalcemia is most often due to hypoparathyroidism after thyroid or neck surgery, vitamin D deficiency, chronic kidney disease and low magnesium. A low calcium diet alone rarely causes it.

What level of calcium is dangerously low?

Corrected total calcium below about 7.5 to 7.6 mg/dL (1.9 mmol/L) is usually treated as severe, because the risk of spasms, seizures and arrhythmias rises sharply. Any level with spasms, seizures, breathing problems or heart rhythm changes needs urgent intravenous calcium, regardless of the number.

Can hypocalcemia be cured?

It depends on the cause. Low calcium from vitamin D deficiency, low magnesium or a medicine usually corrects fully once treated. Temporary hypoparathyroidism after surgery often recovers within weeks to months. Permanent hypoparathyroidism and chronic kidney disease need lifelong treatment to keep calcium stable.

Does eating more calcium fix low blood calcium?

Usually not by itself. Low blood calcium is generally caused by a hormone, vitamin D, magnesium or kidney problem, so treatment targets that cause. Calcium supplements are part of treatment, often with active vitamin D, but diet changes alone rarely correct true hypocalcemia.