What are the possible consequences of failing to treat a parathyroid disorder?

Multiple. If we are dealing with hyperparathyroidism, then untreated it can lead to recurrent kidney stones, osteoperosis, bone fractures, joint pain, fatigue, abdominal pain, and concentration/mood issues too.
Multiple. Possible kidney stones and osteoporosis. Maybe depression. Several studies link hyperparathyroidism to hypertension and coronary heart disease. Find an experienced endocrine surgeon. It is usually a relatively painless outpatient procedure and may give lifetime benefits.
Risks. The risks of untreated Hyperparathyroidism are: Heart disease Increased stroke risk Osteoporosis Kidney stones Brain fog Skeletal pain And many other non specific complaints.