Calcium is a powerful electrolyte that governs numerous processes necessary for life. Although 99% of the body’s calcium is stored in the skeleton, the small fraction circulating in the blood triggers critical functions. This mineral is the primary switch that initiates muscle contraction, allowing the heart to beat and muscles to move. Calcium ions are also fundamental to the nervous system, playing a role in nerve signal transmission and the release of neurotransmitters. Furthermore, calcium is a necessary component in the sequence of events that leads to blood clotting. Because calcium is central to these physiological systems, the body tightly regulates its concentration to maintain balance, known as calcium homeostasis.
Measuring and Defining Normal Levels
Determining a good calcium level involves two common measurements performed during a blood test. Total Serum Calcium is the most frequently ordered test, representing all the calcium in the blood, including both the free and protein-bound portions. For adults, the normal range for Total Serum Calcium is approximately 8.5 to 10.2 milligrams per deciliter (mg/dL).
Ionized Calcium is a more specific measurement that includes only the “free” form of the mineral not attached to proteins like albumin. This free calcium is the biologically active form used by the body’s cells for functions like muscle contraction and nerve signaling. The typical reference range for Ionized Calcium is 4.5 to 5.6 mg/dL.
Ionized calcium is often considered the more accurate measure of true calcium status, especially when blood protein levels are abnormal. Since about half of the total calcium is protein-bound, a low protein level can incorrectly suggest a low total calcium level, even if the active ionized calcium is normal. Therefore, direct measurement of ionized calcium is preferred in patients who are critically ill or have significant protein abnormalities.
When Calcium Levels Are Too Low
A blood calcium level below the normal range is known as hypocalcemia, defined as total serum calcium below 8.5 mg/dL. Common causes include Vitamin D deficiency, which prevents intestinal calcium absorption. Other potential causes are hypoparathyroidism, where the parathyroid glands do not produce enough Parathyroid Hormone (PTH), or chronic kidney failure, which impairs Vitamin D activation.
Symptoms primarily involve increased excitability of the nerves and muscles. A person may experience tingling sensations (paresthesia) around the mouth, fingers, and toes. Muscle cramps, particularly in the back and legs, are frequent complaints.
Severe hypocalcemia can lead to tetany, characterized by muscle stiffness and painful spasms, including throat spasms that cause difficulty breathing. Low calcium levels can also affect the brain, causing neurological symptoms such as confusion, depression, and memory loss. In the most severe instances, hypocalcemia can trigger seizures and dangerous abnormalities in heart rhythm.
When Calcium Levels Are Too High
Elevated blood calcium, or hypercalcemia, occurs when the total serum calcium level rises above 10.2 mg/dL. The two most frequent causes account for over 90% of cases: primary hyperparathyroidism and certain types of cancer. Primary hyperparathyroidism involves an overactive parathyroid gland releasing excessive Parathyroid Hormone (PTH), which pulls calcium from the bones into the bloodstream.
Malignancies, such as lung, breast, or kidney cancer, can raise calcium levels by releasing substances that mimic PTH or by causing extensive bone destruction. Excessive intake of Vitamin D supplements is another cause, increasing calcium absorption from the digestive tract. Symptoms are often described using the mnemonic: “Stones, bones, abdominal moans, and psychic groans.”
Symptoms of Hypercalcemia
- Stones: Increased risk of developing kidney stones and impaired kidney function.
- Bones: Bone breakdown leading to pain, weakness, and increased risk of fractures.
- Abdominal Moans: Gastrointestinal symptoms, including nausea, vomiting, constipation, and abdominal pain.
- Psychic Groans: Effects on the central nervous system, ranging from fatigue and lethargy to confusion, depression, and psychosis.
Hypercalcemia affects nearly every organ system and requires thorough investigation to identify the underlying medical cause.
How the Body Regulates Calcium
The tight control of blood calcium levels involves three major hormones acting on the bones, kidneys, and intestines. Parathyroid Hormone (PTH), secreted by the parathyroid glands, is the main regulator responsible for raising calcium levels when they fall too low. PTH stimulates the release of calcium from the bones, increases calcium reabsorption in the kidneys, and promotes the conversion of Vitamin D into its active form.
The active form of Vitamin D, calcitriol, works alongside PTH by enhancing the absorption of dietary calcium from the gut into the bloodstream. This cooperative action is the body’s primary mechanism for preventing hypocalcemia. Rising calcium levels signal the parathyroid glands to reduce PTH output, forming a negative feedback loop.
In contrast to PTH, the hormone calcitonin, produced by the thyroid gland, works to lower blood calcium levels. It acts by inhibiting the cells that break down bone, reducing the amount of calcium released into the blood. Calcitonin plays a less prominent role in adult calcium regulation compared to PTH and active Vitamin D.