The truth of it is, Vitamin D is more of a hormone than a true vitamin. A vitamin cannot be produced by the body, while vitamin D can be produced by your skin exposure to the sun. Working in concert with the parathyroid glands, Vitamin D plays an important role in the control of calcium. When it is deficient, this balance is lost and the resulting condition is known as secondary hyperparathyroidism, which leads to weakened bones.
A Little Physiology
The parathyroid glands sit on the thyroid gland at the base of the neck and release parathyroid hormone, PTH, in response to low levels of blood calcium. This acts on the bones and kidneys to increase calcium by breaking down bone and reabsorbing calcium from the urine. Finally, PTH also leads to the activation of vitamin D, according to the Linus Pauling Institute, which increases calcium absorption from the gastrointestinal tract. In the setting of high calcium, PTH is decreased and less calcium is transferred to the blood stream from the gastrointestinal tract and the kidneys.
There are three ways in which parathyroid hormone can be over-produced. In primary hyperparathyroidism, the entire parathyroid gland or an adenoma, a type of tumor, releases PTH without any relationship to the blood level of calcium. This leads to high blood calcium. Secondary hyperparathyroidism results from vitamin D deficiency. When there is not enough vitamin D, more PTH is required to have the same effect on increasing the blood level of calcium. In this case, the calcium level may be normal or slightly low. Finally, in tertiary hyperparathyroidism, there is a prolonged period of vitamin D deficiency and the parathyroid becomes so overactive that it behaves as in primary hyperparathyroidism. This condition is mostly seen with chronic kidney failure.
Those at Risk
Anyone can be vitamin D deficient, but certain groups are more at risk. The main risk factors are lack of sun exposure and inadequate oral intake. Individuals with darker skin have a lower production of vitamin D from the skin, particularly in climates farther from the Equator. Elderly patients are also likely to have lower levels due to less sun exposure as well as poor diet.
Diagnosing Hyperparathyroidism Secondary to Vitamin D Deficiency
Typically, hyperparathyroidism symptoms are vague, if present at all, according to American Family Physician. Muscle aches and fatigue are common complaints. This diagnosis is more frequently made either because of screening someone at risk or the calcium level is abnormal. Initial testing involves blood tests for PTH and vitamin D, but only a physician can make a diagnosis of secondary hyperparathyroidism due to vitamin D deficiency.
The treatment of vitamin D deficiency should be individualized, according to the Endocrine Society's Clinical Guidelines. In a person without kidney disease, vitamin D can be replaced with prescription ergocalciferol or over the counter cholecalciferol. Patients with chronic kidney disease often require supplemental activated vitamin D, as they are less able to make the active form. The relationship between vitamin D and parathyroid hormone is complex and it is best if a medical professional is involved in making the diagnosis and recommending treatment.