Why is it important? Because it looks a lot like primary hyperparathyroidism. A significant number of cases per year get confused with hyperparathyroidism and as a result undergo inappropriate (and failed) surgery. It’s very tricky. There are ways to make the distinction, as outlined in the articles. If you are about to diagnose primary hyperparathyroidism and refer a patient for surgery pause and ask yourself: am I missing familial hypocalciuric hypercalcemia?