Dementia is diagnosed when a person has lost significant mental function, defined as the ability to think, reason and remember. A diagnosis of dementia can be frightening, but many people do not realize dementia can be reversed in some cases.

Medication

Some prescription medications and some combinations of prescription drugs can cause dementia symptoms. The dementia is reversible when the drug or drug combination is discontinued.

Low Thyroid

Low thyroid function, also known as hypothyroidism, can lead to confused thinking that mimics dementia. Treating the low thyroid condition will reverse the dementia symptoms.

Head Injury

Head injury can occur during a fall or other trauma to the skull, and it is a common cause of potentially reversible dementia in the elderly. Rehabilitation can improve and even restore cognitive function completely.

Nutritional Deficiencies

Deficiencies in certain vitamins, notably vitamins B1, B6 and B12, can lead to symptoms of dementia. If these deficiencies are found through medical tests and corrected, the dementia can reverse.

Infections

Some infections, including meningitis, Lyme disease and encephalitis, also can lead to delirium and symptoms of dementia. Patients with immune systems that are compromised from AIDS or leukemia may develop an infection called progressive multifocal leukoencephalopathy (PML). Treatment of these infections can reverse dementia symptoms.

Source:

MedicineNet: Dementia causes

DementiaGuide: Reversible causes