Calcium is a mineral essential for maintaining the body’s balance. While over 99% of the body’s calcium is stored in the bones for structure and strength, the small fraction circulating in the blood is necessary for numerous functions. This extracellular calcium facilitates the electrical signaling required for proper nerve function and all forms of muscle contraction, including the beating of the heart. The body tightly regulates the concentration of calcium in the bloodstream to ensure these processes occur without disruption.
Defining the Reference Range
The reference range for blood calcium relies on two distinct measurements, both reported in milligrams per deciliter (mg/dL). The most common test is Total Serum Calcium, which measures all calcium in the blood, including the free, active form and the portion bound primarily to albumin. For adults, the typical range for Total Serum Calcium is between 8.5 and 10.5 mg/dL.
A more precise measurement is the Ionized Calcium test, especially when protein levels are altered. This test measures only the “free” calcium, which is the biologically active form immediately available to tissues. Approximately half of the calcium in the blood is in this free, ionized form. The normal range for Ionized Calcium generally falls between 4.6 and 5.3 mg/dL for adults, and it is considered the most accurate indicator of the body’s true calcium status.
Hormonal Mechanisms That Maintain Stability
Maintaining calcium levels within this narrow range is achieved through a feedback loop involving two primary hormones. Parathyroid Hormone (PTH), secreted by the parathyroid glands, is the main regulator that responds directly to low calcium levels. When blood calcium begins to drop, PTH is released, triggering actions to restore the balance.
PTH acts on the bones to stimulate the release of stored calcium and signals the kidneys to reabsorb more calcium instead of excreting it in urine. Additionally, PTH promotes the conversion of Vitamin D into its active form, calcitriol, within the kidneys. Calcitriol then works primarily on the intestines, significantly increasing the absorption of calcium from digested food into the blood.
Causes and Effects of Elevated Calcium
When blood calcium levels rise above the normal range, hypercalcemia occurs, which can weaken bones and affect the function of the kidneys and nervous system. The most frequent cause is primary hyperparathyroidism, where the parathyroid glands produce excessive PTH, leading to uncontrolled calcium release. The second most common cause is malignancy, as certain cancers can secrete a protein that mimics PTH action or directly cause bone breakdown.
Symptoms are often subtle in mild cases but become more pronounced as levels increase. Patients may experience gastrointestinal issues, including constipation, nausea, and abdominal pain. Excess calcium makes the kidneys work harder, leading to increased thirst and frequent urination, which can cause dehydration. Over time, hypercalcemia can lead to the formation of calcium crystals in the kidneys, resulting in painful kidney stones.
Neurological symptoms range from fatigue, muscle weakness, and depression to more severe effects on the brain. High calcium levels decrease the excitability of nerves and muscles, potentially causing confusion, forgetfulness, and, in severe cases, stupor or coma. Prolonged hypercalcemia can also contribute to bone thinning, known as osteoporosis, as calcium is continually pulled from the skeleton.
Causes and Effects of Depleted Calcium
A blood calcium level below the normal range is defined as hypocalcemia, resulting from issues with calcium regulation or absorption. One major cause is hypoparathyroidism, a reduced function of the parathyroid glands leading to insufficient PTH production, often following neck surgery. Other common causes include Vitamin D deficiency, which prevents calcium absorption from the intestines, and chronic kidney disease, which impairs Vitamin D activation.
The effects of hypocalcemia are primarily related to increased neuromuscular excitability. Low calcium levels lower the threshold for nerve and muscle cell firing, leading to characteristic symptoms. Patients often report paresthesia, a tingling or numbness sensation typically felt around the mouth, fingers, and feet.
As the condition worsens, increased excitability can lead to muscle cramps, spasms, and tetany, which involves involuntary muscle contractions. Tetany can manifest as carpopedal spasm, where the wrist flexes and the fingers draw together, or spasms of the throat muscles, causing difficulty breathing. Long-term hypocalcemia can also affect the brain, potentially causing confusion, memory loss, and depression.