Primary bone disorders are diseases that directly affect bone density and strength. An example of a primary bone disorder is osteogenesis imperfecta.
Secondary low bone density is caused by underlying medical conditions. Examples include female athletes not getting their period, anorexia nervosa, other conditions of low estrogen, Crohn’s disease, celiac disease, cystic fibrosis, Cushing’s syndrome, conditions requiring chronic steroid treatment, and conditions where the child has limited mobility.
Idiopathic low bone density occurs without a clear cause.