Low Vitamin D Levels
Vitamin D helps our intestine absorb calcium from the foods we eat and increase the amount of calcium in our bodies. It’s also needed for bone growth, and remodeling. Vitamin D is essential to our body’s proper function; we can produce it by being exposed to sunlight or from our diet.
If vitamin D levels are low, the intestines will struggle to absorb calcium. This is actually why milk, a rich source of calcium, is often fortified with vitamin D. Without vitamin D, most of the calcium in the milk will not get absorbed. Increasing an individual’s vitamin D levels will simply increase the amount of calcium they absorb from their diet. If a person takes more vitamin D, the intestines will become more efficient at absorbing the calcium molecules in our diet, and your calcium level will go up.
Hyperparathyroidism is associated with high calcium in the blood, which the body does not want. As a result, our bodies will try to get rid of the excess calcium in the urine, explaining why every third patient will have high calcium in the urine. The body will also want to shut down calcium absorption from your intestines, and it does this by limiting the amount of vitamin D in your body. If your calcium level is too high, the body will decrease the amount of vitamin D in order to decrease the amount of calcium that is absorbed from your intestines. Because vitamin D is needed to absorb calcium from your diet, the decreased levels will restrict the amount of calcium your body can absorb.
Now this is when vitamin D becomes confusing. There are cases of secondary hyperparathyroidism, with a low vitamin D causing low calciums and therefore an increase in PTH levels, but these are rare cases. More often you will find high PTH levels, and high calcium levels and low vitamin D, in a patient with primary hyperparathyroidism. As compared to people with primary hyperparathyroidism and normal vitamin D levels, the ones with primary hyperparathyroidism and low vitamin D have:
- Higher PTH levels
- Higher markers of bone turnover
- Larger parathyroid adenomas
- More frequent fractures
A study by Dr. Bilezikian (published in Journal of Clinical Endocrinology and Metabolism in 2009) showed, in these patients with primary hyperparthyroidism and low vitamin D levels, taking vitamin D to normalize vitamin D can lower the PTH, improves bone density, and it does not worsen hypercalcemia (meaning it wont increase the calcium in the blood or urine). This has to be done with caution and close monitoring, because it is not uncommon for patients who are on vitamin D therapy when they have primary hyperparathyroidism to feel ill when they are taking vitamin D replacement in which case it should be stopped.
However, it is important to remember that measuring vitamin D levels has nothing to do with diagnosing hyperparathyroidism. If your calcium is high, you almost certainly have hyperparathyroidism. If your calcium is high and your vitamin D is low, you still have hyperparathyroidism. But while low vitamin D levels will never cause high calcium levels, removing a parathyroid tumor will almost always correct the vitamin D level.