Normocalcemic primary hyperparathyroidism (nPHPT) is a type of primary hyperparathyroidism that has been increasingly discovered in patients, yet cannot be detected through high blood calcium lab results. Instead, patients with nPHPT are often uncovered during work up for osteoporosis or low bone mineral density. In nPHPT, the patient’s calcium level stays within the normal range but parathyroid hormone levels are too high. (See Normal Calcium Levels) To properly diagnose these patients, more work up is required to rule out other causes of elevated PTH levels.
Ruling out secondary hyperparathyroidism (Vitamin D deficiency, kidney function)
To diagnose nPHPT, we are most interested in the inappropriate relationship between the parathyroid glands and blood calcium levels, so we must rule out secondary hyperparathyroidism. In secondary hyperparathyroidism, the parathyroid gland is functioning normally and is only producing high levels of PTH because it is doing its job in compensating for other conditions in the body.
Patients with low levels of calcium and high levels of PTH have secondary hyperparathyroidism. In fact, the parathyroid gland is functioning correctly since it is producing high levels of PTH in order to bring the blood calcium levels back up to the normal range. This may be due to a malabsorption problem such as celiac disease.
Patients with normal levels of calcium, high levels of PTH, and vitamin D deficiency also have secondary hyperparathyroidism. Vitamin D is required for the body to absorb calcium from food. The normal range of Vitamin D is 30-80 ng/mL. With low levels of vitamin D, patients are not able to absorb calcium correctly and therefore the blood calcium is low. Again, the parathyroid glands correctly increase PTH levels in order to increase blood calcium. After vitamin D supplementation, if the patient’s PTH level returns to normal, we can confirm that it was secondary hyperparathyroidism. If after Vitamin D supplementation the patient’s calcium levels become high and the PTH levels remain elevated, we would have uncovered primary hyperparathyroidism.
Ruling out Chronic Kidney Disease and Urinary Calcium Leaks
Kidney failure, or an eGFR of less than 60, causes a problem with vitamin D metabolism. This also leads to the parathyroid glands appropriately compensating the loss of blood calcium by increasing PTH levels. Calcium in the blood can also be lost as blood filters through the kidneys and too much calcium leaks into the urine (called urinary calcium leak). Excess calcium in kidney filtration can lead to kidney stones.
|Primary Hyperparathyroidism||Normocalcemic Primary Hyperparathyroidism (nPHPT)||Secondary Hyperparathyroidism|
|Blood Calcium||High||Normal||Normal or low|
|Vitamin D||Usually normal||Must be normal||May be low|
|GFR (kidney function)||Usually normal||Must be normal||May be low|
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