The big picture: Calcium is the most abundant mineral that is essential for many functions in the body, including muscle function, nerve transmission, blood clotting and the formation of bones and teeth. Abnormal levels indicate possible bone, kidney, thyroid, or parathyroid disorders. Low levels increase your risk for stress fractures and high blood pressure.

About 99% of calcium is in bones and teeth and the other 1% is found in blood and soft tissue. A calcium blood test cannot show how much calcium you have in your bones.

Each day, some calcium is naturally lost, filtered from the blood by the kidneys, and excreted through urine and sweat.
Measuring calcium in the urine helps assess how much calcium is being eliminated by the kidneys.

Calcium blood levels are tightly and mainly controlled by the parathyroid hormone (PTH), which is primarily produced by the parathyroid glands located behind the thyroid in your neck.

PTH is released in response to low blood calcium levels and works to increase these levels by enhancing calcium absorption in the gut and kidneys and releasing calcium and phosphate from the bones - a process known as bone resorption.
Vitamin D also plays an important role in maintaining calcium levels because your body needs it in order to absorb calcium. Without sufficient vitamin D, dietary calcium is not efficiently absorbed, leading to lower calcium levels in the blood.

Too little calcium, also known as hypocalcemia, is most commonly caused by PTH deficiency or vitamin D deficiency or resistance. Most people with chronic hypocalcemia are asymptomatic.

Hypocalcemia can lead to muscle weakness, fatigue, confusion, depression, and memory loss.
More severe symptoms, typically in acute hypocalcemia, include seizures, muscle spasms, numbness and tingling, spasms of the larynx, anxiety, and heart rhythm abnormality.

Too much calcium, also known as hypercalcemia, is most commonly caused by PTH excess (hyperparathyroidism) due to enlargement of one or more of the parathyroid glands either benignly or due to cancer.

Often chronic and mild; many patients show no symptoms.
Manifestations are a function of its severity and rapidity of onset. Early symptoms can include fatigue, weakness, anxiety, increased sleepiness - followed by nausea, vomiting, abdominal pain and constipation.