How Does Vitamin D Affect The Parathyroid?

Calcium is the most abundant mineral in your body. According to the Linus Pauling Institute, 99 percent of your calcium stores are sequestered in your bones and teeth at any given time, with the remainder found in your soft tissues and bloodstream. Calcium is continuously exchanged between your bone and soft tissue-blood reservoirs, but this calcium flux is meticulously controlled by your parathyroid glands, which are under the influence of vitamin D.

Parathyroid Glands

Your parathyroid glands are situated behind your thyroid, which is located just beneath your "Adam's apple." Most people have four parathyroid glands, but the number can vary from two to six. Your parathyroid glands secrete parathyroid hormone, or PTH, in response to falling blood calcium levels. "The Merck Manual of Diagnosis and Therapy" reports that PTH can increase your blood calcium level within minutes by stimulating the absorption of calcium from your intestines and kidneys and by mobilizing calcium and phosphate from your bones. Under the influence of PTH, your kidneys then eliminate the excess phosphate.

Vitamin D and Parathyroid Function

Like nearly every other tissue in your body, your parathyroid glands possess vitamin D receptors, or VDRs. Vitamin D stimulation of parathyroid VDRs switches off parathyroid hormone production. As PTH levels fall, absorption of calcium from your intestines and kidneys decreases, and mobilization of calcium and phosphate from your bones ceases. This allows your blood calcium levels to fall. However, vitamin D also enhances intestinal and renal calcium absorption, which maintains your blood calcium levels despite a decrease in PTH.

Vitamin D Deficiency

The maintenance of a normal serum calcium level is essential for normal muscle, nerve and cardiovascular function. In fact, your body will sacrifice your bones, if necessary, to ensure normal blood calcium levels. If you become vitamin D-deficient, your parathyroid glands produce more PTH to maintain your blood calcium. As PTH levels rise, the turnover of calcium and phosphate from your bones increases. This helps to maintain normal blood calcium levels, but the phosphate released from your bones is excreted in your urine. Without sufficient phosphate, your bones cannot be mineralized, and bone strength declines. The April 1999 issue of "Alternative Medicine Review" reports that this state of "secondary hyperparathyroidism" eventually leads to osteoporosis and increases your risk of fractures.

Delicate Balance

Your calcium status is meticulously controlled by a complex interplay between your vitamin D status and your parathyroid hormone level. Rising blood calcium levels and increased vitamin D availability suppress the production of parathyroid hormone, thus moderating any increase in blood calcium. Conversely, a drop in either your blood calcium or vitamin D levels prompts an increase in PTH secretion, which accelerates bone turnover and increases intestinal and renal absorption of calcium.

Sources

Sun exposure or consumption of vitamin D-containing foods helps to maintain your calcium balance and protect skeletal integrity. Because melanin blocks the sun's rays, lighter-skinned people produce more vitamin D when exposed to sunshine than darker-skinned people. The Office of Dietary Supplements suggests exposing your face, arms, legs or back -- without sunscreen -- for 5 to 30 minutes twice weekly between 10 a.m. and 3 p.m. Alternatively, you can get vitamin D from oily fish such as salmon, mackerel, sardines or tuna; fortified foods, such as milk and breakfast cereals; or supplements.

Recommendations

Guidelines for dietary intakes of vitamin D vary, depending on your age and pregnancy status. The Food and Nutrition Board recommends that infants to 12 months of age receive 400 IU daily, while older children, pregnant women and adults to age 70 receive 600 IU daily. Adults over the age of 70 should have 800 IU each day to help protect bone integrity and prevent osteoporosis.