Calcium makes up approximately 1 to 2 percent of your body weight. About 99 percent of the calcium is found in bones and teeth and 1 percent is found in the fluid inside and outside individual cells, according to "Advanced Nutrition and Human Metabolism." The 1 percent of calcium found outside of the bones is strictly regulated in the body by hormones. The production and secretion of these hormones determine how the remaining calcium is absorbed and utilized in the intestines, kidney and bone. One of the regulating hormones is calcitonin, which is produced in the thyroid gland.
Calcium Sources and Absorption
Calcium food sources include dairy products and fish like salmon and sardines that are served with bones. Dietary supplements such as calcium carbonate, calcium acetate and calcium citrate also provide the body with calcium. The body absorbs calcium mainly in the small intestine, but according to "Advanced Nutrition and Human Metabolism," the colon also absorbs 4 to 10 percent of dietary calcium per day. Vitamin D is also essential for optimal calcium absorption.
Roles of Calcium
The most well known function of calcium is aiding in the mineralization of bones with the help of phosphorus, fluoride, magnesium and potassium. However, it is the role of the lesser-known "non-bone" content of calcium that is responsible for how much calcium is absorbed, transported and excreted. Calcium not in the bones is responsible for a number of actions that include blood clotting, nerve conduction, muscle contraction and enzyme regulation.
Calcium-Thyroid Connection
Calcitonin, along with the parathyroid hormone and calcitrol, tightly regulates how much calcium is in the blood. The thyroid gland produces calcitonin that works in contrast to the parathyroid hormone, which is produced in the parathyroid gland, and calcitrol, which is produced primarily in the kidneys. Too much serum calcium will stimulate production of calcitonin in the thyroid and tell the body to stop the activity of bone cells called osteoclasts from releasing more calcium into the blood. Calcitonin also stops calcium from absorbing into the kidneys by stopping production of vitamin D, thus excess calcium is excreted from the body in urine.
Interference with Thyroid Medication
An indirect connection between calcium and the thyroid gland is levothyroxine, a synthetic hormone medication taken for the treatment of an under active thyroid gland. The "Journal of the American Medical Association" published a clinical investigation in 2000 that concluded calcium interferes with the absorption of levothyroxine in hypothyroid patients. It is estimated that absorption of levothyroxine can decrease by one third in some patients when ingesting calcium-containing foods or supplements when taken at the same time.
References
- "Clinical Endocrinology and Metabolism"; Disorders of calcium homeostasis; G. Carmelliet et al; 2003.
- "Vitamin D Council"; Vitamin D Council Glossary:
- "Journal of the American Medical Association"; Effect of calcium carbonate on the absorption of levothyroxine; N. Singh et al; June 2000.
- Drugs.com; Drug interactions between calcium carbonate and Synthroid;
- "Advanced Nutrition and Human Metabolism, Fifth Edition"; Sareen Gropper et al, 2009
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