Calcium is the most abundant mineral in the human body and has several important functions. More than 99% of total body calcium is stored in the bones and teeth where it functions to support their structure. The remaining 1% is found throughout the body in blood, muscle, and the fluid between cells. Calcium is needed for muscle contraction, blood vessel contraction and expansion, the secretion of hormones and enzymes, and sending messages through the nervous system. A constant level of calcium is maintained in body fluid and tissues so that these vital body processes function efficiently.

Bone undergoes continuous remodeling, with constant breakdown of bone and deposition of calcium into newly deposited bone (bone formation). The balance between bone breakdown and bone formation changes as people age. During childhood there is a higher amount of bone formation and less breakdown. In early and middle adulthood, these processes are relatively equal. In aging adults, particularly among postmenopausal women, bone breakdown exceeds its formation, resulting in bone loss, which increases the risk for osteoporosis. 

Calcium will absorb better with vitamin D, which you can get from sunlight, some foods or a supplement. Several calcium supplements have Vitamin D added. Another important ingredient in a good calcium supplement is magnesium. Magnesium and calcium work together in several bodily functions including muscule contraction and energy. Whereas calcium contacts the muscles, magnesium is needed to balance the effect and relax them. Magnesium in a calcium supplement should be in a form other than "oxide" as this form does not absorb in the body very well. Calcium citrates or a combination calcium will absorb better than calcium carbonate, and can be taken on an empty stomach, if needed.

Do not take calcium supplements at the same time as thyroid medication, as it may inhibit the absorption of the thyroid medication.

Separate the calcium supplement and thyroid medication by at least four hours.