Vitamin D deficiency is common in patients with primary hyperparathyroidism.
The parathyroid glands regulate calcium levels in the blood, and part of how they do that is by activating Vitamin D. Vitamin D exists in multiple forms, and the inactive form gets converted into the active form in the kidneys. Parathyroid hormone (PTH) is what causes that conversion. Active Vitamin D helps the intestines absorb calcium, raising blood calcium levels.
When there is too much PTH, as in the case of primary hyperparathyroidism, much of the inactive Vitamin D is made into active Vitamin D. Patients end up with high levels of active Vitamin D, but low levels of inactive Vitamin D. The active form contributes to the high calcium levels seen in primary hyperparathyroidism. Taking more Vitamin D will just make the problem worse, because it could raise calcium levels further.
Unfortunately, most of those patients are diagnosed with a Vitamin D deficiency, even though their active Vitamin D levels are high. Why? Physicians typically only measure inactive Vitamin D, called Vitamin D 25-OH, when checking vitamin levels. If only the inactive level is checked, it looks like the patient has a deficiency, prompting the physician to prescribe Vitamin D supplements.
It is possible to check levels of the active form of Vitamin D, called Vitamin D 1,25-diOH. But we don't normally do it. For most people, the inactive form is a better measure of long-term overall Vitamin D status. The inactive form sticks around in the body longer while the active form breaks down quickly, so its levels fluctuate more. In healthy people, the inactive form tends to correlate closely to the active form, so just checking one is good enough, and we would prefer to check the one that has more stable levels over time.
The inactive Vitamin D provides an accurate picture of Vitamin D status in most people, with one exception: those with primary hyperparathyroidism. In these patients, the inactive form is being converted to the active form, giving the impression of a Vitamin D deficiency. To get a real sense of Vitamin D status, the active form would also need to be checked - and in primary hyperparathyroidism it is usually high.
Patients with high calcium levels (characteristic of primary HPT) do not need Vitamin D supplementation. Vitamin D helps the intestines absorb calcium, and supplementation can raise blood calcium levels. If the calcium is already high, this can worsen the condition. After parathyroidectomy, the Vitamin D level will normalize. Supplementation is not necessary if the diagnosis of primary hyperparathyroidism is clear.
After parathyroid surgery, the story is different. Every patient should take Vitamin D after having parathyroid surgery. When the calcium level drops after surgery (which happens if you are cured), you need to take calcium and Vitamin D to stabilize the calcium levels. Vitamin D is essential for this.