Is my cat crazy, or is there a reason they are licking and itching until their hair falls out?

Years ago, it was thought that about 70% of overgrooming in cats was due to a psychogenic issue.  Now we know that more than 70% of these cases actually are itchy from allergies.  The second most common reason for this type of behavior is due to parasites:  fleas or mites.  If there is exposure to the outdoors or a kitten or new cat recently, a type of fungal infection called ringworm must be ruled out as well.  Many cats with this behavior are trying to remedy an internal pain or metabolic disease (inflammatory bowel disease, hyperthyroidism, or urinary tract disease are most common).  Only a small percentage are licking and pulling hair like this as a purely psychogenic manifestation.  Scratching the head or biting at the feet rules out psychogenic causes and lets you know the cat is definitely itchy.
The diagnosis of allergies involves ruling out all other causes of itchiness and responding to treatments for allergies. Like dogs, cats can have different types of allergies:  food, fleas, and environmental types.  Food allergies cannot be differentiated from environmental allergies by symptoms alone, a food trial must be performed with a home-made diet or a commercial prescription food. There are several medications to treat allergies in cats.  In addition, allergy testing and desensitization can be performed.  So if your cat is losing hair, we can help rule out other causes and formulate a relief-giving treatment tailored for your suffering cat.