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I am sorry to hear you are worried about Clancy. My name is Rebecca. I am a veterinarian and will do my best to help.
Is he itchy in the areas of hair loss?
Is the area of hair loss totally smooth and hairless, or are there short broken hairs there?
How old is Clancy? Is he the only cat in the house?
Sorry for more questions! Since I can't see and examine him, the more information you give me the more I can help.
If he is itchy, this may be some kind of allergic reaction. I know they are indoor cats, I know you don't see fleas, but you still might consider trying Frontline or Advantage for a month. I will never forget the indoor only totally bald cat I knew years ago. He wasn't even itchy so we never suspected fleas. When Advantage came out (so this was like 20 years ago), they put it on him since they had a dog that went out and lo and behold! All his hair grew back! He had been bald for years, and it was flea bite allergy all along. We never saw a flea on the cat, and since he was bald you would have thought we would have at least seen one or two! Since Frontline is so safe (not absorbed into body or bloodstream, it would be an easy thing to try.
Hormone imbalances can cause hair loss on the hind end in a symmetrical pattern, but is usually not itchy.
You can try treating for allergies by giving him 5 to 10 mg Benedry twice daily or 2.5 mg Claritin once daily.
Melatonin can help hair to regrow sometimes: 3 mg once to twice daily.
How old is he? Is he neutered?
I did not hear back from you; how is Clancy doing? I hope you were able to see my response. Let me know if this was helpful and if I can answer any more of your questions.