Calcium has many essential functions in the body and important for bone formation and strength, proper muscle and nerve function, and blood clotting. If calcium levels are low or high, a hormone imbalance may be present. Possible causes of low calcium levels include hypoparathyroidism (decreased parathyroid hormone levels), vitamin D deficiency (low levels of vitamin D in the body) or vitamin D resistance (inability to respond to vitamin D in the body), certain medications, or temporary surgical effects. Possible causes of high calcium levels include vitamin D excess, hyperparathyroidism, certain medications, or a separate underlying medical condition. Specialized testing may be needed to determine if any of these conditions are present and require treatment.