As a houseofficer many years ago, I remember that there are two electrolytes that are frequently encountered during clinical practice- Potassium and Calcium.
We have discussed a lot about Potassium, I am going to talk about Calcium metabolism today and of course talk more about hypercalcemia.
It is pretty easy to remember, the only pool of calcium in our body is bone. Although tiny amount of calcium is being absorbed through the gut ( affected by Vitamin D), maintenance of normal calcium level in serum ( 2.2-2.6) greatly depends on exchange of Calcium between extracellular fluid and bone.
It is easy to remember that if we have low calcium level, our body will try to do the followings to increase calcium level in the serum,
1) Increase Calcium absorption from the gut
2) Increase bone resorption in the bone so that more calcium can be released to the serum
3) Reduce Calcium excretion from the kidney
The main organ that regulates these is parathyroid hormone. You can think of causes of hypercalcemia into a few big groups as below,
1) Bone problem
It is easy to understand this, when there is increased bone destruction, of course you calcium level is high. Therefore, any malignant disease either primary or secondary that leads to bone destructions can cause hypercalcemia.
2) Vitamin D problem
As I said before, Calcium absorption from the gut is mainly affected by Vitamin D, therefore, Vitamin D toxicity or granulomatous diseases ( such as Sarcoidosis or tuberculosis) can cause hypercalcemia.
3) Parathyroid hormone
Of course, when you have high parathyroid hormone ( primary and secondary), you calcium level is high but remember that secondary hyperparathyroidism may have normal or even low Calcium level.
4) Others
Some other rare causes such as Familial hypocalciuric hypercalcemia, milk alkali syndrome, immobility etc.
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