What may cause slightly below norm blood calcium levels over multiple years in a f 27yo with osteoperosis but good calcium intake & sun exposure?

Complex!: A 27 y.o. with osteoporosis immediately raises the concern for further evaluation. Given a history of an eating disorder if this is still present the Ca on lab reports may not be accurate as the level is effected by Albumin levels which are often low in people with eating disorders causing the Ca to appear low. Can check free Ca. Also need to look at Phos, Mag, Vit D (make sure on enough), iPTH.