Can undiagnosed diabetes pose a threat to your health in the long run?

Yes, Gigantic: From population studies, at time of diagnosis, hba1c ≥6.5%, people with elevated glucose are not just starting to more rapidly develop atherosclerosis, they have been doing so for several decades, do not live as long & have more cardiovascular events than people without dm who have already had a heart attack. Plus dm have problems with a vast array of body functions, slow healing, etc.