The diagnosis of deficiency/insufficiency is given when an individual has levels < 30 ng/mL because reaching sufficiency status requires an increase in PTH correlated with the reduction in vitamin D serum levels; thus, deficiency is defined by the level of 25 (OH) D below 20 ng/mL and insufficiency between 21 and 29 ng/mL [89,93]. Here, PTH is linked to hyperinsulinemic hypoglycemia, familial, 4.