Vitamin D exists in several forms, but the active form in the human body, calcitriol, helps regulate calcium levels. By increasing calcium absorption from the intestine, calcitriol plays a role in proper bone formation. High blood levels of vitamin D are rare, and almost always result from exorbitant oral intake, which in turn leads to elevated calcium levels. Other causes of high blood calcium include overactive parathyroid glands and cancer.
Symptoms
In isolation, too much vitamin D does not cause symptoms, although it may lead to high blood calcium levels, or hypercalcemia, which can be responsible for a myriad of symptoms. Gastrointestinal symptoms often predominate, and they may include loss of appetite, nausea, vomiting, constipation and abdominal pain. Kidney impairment may present with increased urination, increased thirst, dehydration and kidney stones. Calcium has an important role in muscle function, and excess levels may lead to twitching and weakness of the limb muscles. In addition, abnormal heart rhythms may result. The brain is sensitive to changes in calcium levels, and an excess can lead to confusion, irritability, memory loss and delirium. Coma or death may result from very high levels of calcium.
Signs
Measurable effects of too much calcium and vitamin D include high blood values of both substances. Blood measurements of calcium and various forms of vitamin D, associated with symptoms, confirm the diagnosis. Abnormalities on urinalysis may reflect calcium's effects on the kidneys, as the kidneys begin to excrete increased amounts of protein and calcium. Kidney stones may be visible on an X-ray or CT scan. An electrocardiogram measures the electrical activity of the heart and may demonstrate changes related to high calcium levels.
Complications
The classic medical school rhyme for problems associated with excess calcium levels is "stones, bones, abdominal moans and psychic groans." This mnemonic relates to kidney stones, calcium deposits in joints -- termed pseudo-gout -- pain from pancreatitis or peptic ulcer disease, and psychosis. As nearly all cases of vitamin D toxicity result from excess intake, you should exercise care when taking prescription or over-the-counter supplements.
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