When you have high calcium, it does not make sense to take Vitamin D supplements, because this can raise your calcium further. One of the main roles for Vitamin D is to help your intestines absorb calcium, thus increasing your blood calcium level. If your blood calcium level is already high, you don’t want to take Vitamin D and raise your calcium further.    Patients with primary hyperparathyroidism often are told that they have low Vitamin D levels. They often come with a diagnosis of “Vitamin D deficiency.” But Vitamin D exists in multiple forms in your body. The one we most commonly measure is Vit D 25-OH. This is an inactive form, which gets converted in the body to the active form, or Vit D 1,25-diOH. The enzyme that converts the inactive form to the active form is stimulated by parathyroid hormone (PTH). When you have a parathyroid tumor, which produces too much PTH, your inactive Vit D is converted to active Vit D (which we usually do not measure). If you measure this active form of Vit D in parathyroid patients, it is often high. But the inactive form is usually low. Since that is the form we measure, patients are diagnosed with deficiency.   Most doctors will not remember this about Vitamin D and PTH (it’s not something they really need to know for most patients). They see a low Vitamin D and they automatically prescribe Vitamin D supplementation. But if that “low Vitamin D” is actually due to a parathyroid adenoma (benign tumor), it should not be treated. The treatment for this is removal of the parathyroid adenoma. Your Vitamin D level will improve after parathyroid surgery. And after the operation, you should be taking calcium and Vitamin D, but not before.