This was news to me, but quite a few of us may have had a traumatic brain injury in the past - even "just" concussion and not made the link.
Those head injuries can damage the tiny pituitary gland, I think in about a third of cases.
Damage to the pituitary can lead to OP. Search hypopituitarism and osteoporosis to discover more. And of course it can be treated.
Those who have had a rheum. consult will likely have been tested, but those not yet bad enough to be referred might well be missed.