Hypocalcemia describes lower than normal levels of calcium in the blood. This can be caused by abnormally low protein levels (as calcium binds freely to some proteins in the blood), but can also be caused by numerous other conditions. To rule out protein related conditions, the level of ionized calcium in the blood is usually treated as the more accurate figure.
A differential diagnosis would start with low thyroid hormone levels, then Vitamin D deficiency. It can also be caused by either high or low magnesium levels. It can more rarely be caused by eating disorders, such as anorexia nervosa and bulimia, mercury poisoning, and tumor lysis syndrome.