If severe osteoporosis which is secondary was left untreated, what is the worse that can happen? and why would a doctor not treat it?

Fractures: The worst that could happen is a hip fracture which can be very difficult to heal and limits mobility greatly or compression fractures of the spine which leads to chronic pain. It is difficult to say which it was not treated. You should discuss with your dr. And consider seeing a specialist like an endocrinologist to discuss treatment and risk/benefits of treatment.

Fractures.: The ultimate concern with osteoporosis is fractures of the bones. Hip fractures, for example, are associated with significant morbidity and mortality. With osteoporosis you can break any bone more easily from falling or just hitting into something hard.