Learn about five illnesses that might be confused for depression, starting with the one that Amanda Cullinan and her physicians couldn’t figure out. ADHD. The first image that comes to mind when you hear the acronym ADHD is usually of a hyperactive little boy who can’t sit still. Hypothyroidism is a condition in which the thyroid gland is underactive. Fibromyalgia and number four Lyme disease is a kind of tick-borne illness.