Have the hormones tested.  Then, supplement whatever hormones are low.  I would get natural cat hormones if available, otherwise your cat can get cancer.  Spaying or neutering an animal is no different than doing it to a person.  And just doing a tubal on a female, person or animal, often leads to  PTLS.  When animals get spayed or neutered (especially when very young), they often experience skin problems, which are hormone related (and probably many other ailments they cannot tell us about).  When done young (under a year), it can lead to underdeveloped organs and they can have symptoms associated with that.  Also, it wouldn't hurt to run a full metabolic panel to make sure the thyroid and the rest of the system is functioning ok. 

Whenever a vet says: "he's allergic to fleas" hear:"he's gluten-intolerant or possibly allergic to the dyes and other crap in most pet food" Did the vet test him for allergy to fleas? I doubt it.Whenever they see hair loss and scratching thats their answer.Buy some petguard (my favorite) or myrick's or newmans own-might have to go to the health food store.Go to dogtorj.net.Even the idiots with the md degrees will tell you the primary allergen to people and animals is high gluten wheat(thanks to the conquerers of the romans)which caused the dark ages,dairy (replacement of goats milk and cheese with that of the cow and the insuing crash of human immune systems that led to the great plagues of europe)&corn.The crash of the US corn market when china started growing their own and its proliferation into pet food in the 70's led to the decimation of two great breeds of dogs' the irish setter & the golden retriever,highly gluten intolerant breeds.And now sow.Better than a steady diet of grease but full of gluten and phytoestrogens.but here I go again. Get some healthy pet food.