Calcium is the most abundant mineral in the body, essential for bone and tooth structure, muscle contraction, nerve signalling, blood clotting, and hormone secretion. About 99% is stored in bones; the remaining 1% circulates in blood where it must stay within a narrow range for normal physiological function.

Blood calcium is regulated by PTH, active vitamin D, and calcitonin. Total calcium is routinely adjusted for albumin, since a significant proportion circulates bound to albumin and low albumin can produce apparent hypocalcaemia in the absence of true deficiency.