FNDC5 and vitamin D deficiency: Therefore, we measured irisin serum levels in a cohort of patients affected by primary hyperparathyroidism (PHPT) treated for 6 months with vitamin D. In PHPT, a pathological condition caused by an adenoma of the parathyroid, vitamin D insufficiency is more common than in the general population and, although its supplementation is still debated, lower vitamin D levels are associated with more severe disease [14,15,16,17].