Please take him to the vet lets make sure the original surgical procedure was done properly and he is not producing any feral kittens.

I'd also be concerned at his age of hyperthyroidism. Although he would be the first I have heard of to exhibit those particular new behaviors, I have heard of cats demonstrating interesting behavior changes because of that disease (more clingy, more aggressive, up all night is common).

I had the same thought as Meg. HyperT causes all body systems to go into overdrive, so if he has even the slightest testicular tissue left in his body somewhere, it may be manifesting these behaviors as a result of the hyperT.

Laurie

They may have left some testicular material behind, not fully neutered. I had a male that had a testes (one) that was not fully descended, the vet warned me before hand that he may not be able to get everything. He really needed to get everything. He did, but it was about 6 inches up his chest area. He had stag tail, it's really awful, waxy stinky secretions, he was a persian so it was next to impossible to groom the poor thing.

Many thanks indeed for all your comments. When I first emigrated and took him to a local vet when he was due for refresher jabs, the vet said 'have you thought about having him neutered?' and could hardly believe me when I said I had him done in England at 6 months old. The vet said he hadn't come across a neutered tom that big or strong before. He is very big with muscle, not fat, and that is the sort of cat I thought I had. He has picked fights with 'complete' toms many times now and emerged as the victor.

Size doesn't necessarily equate with or relate to neutering. My boy, Noddy, is the largest cat I've ever known, and he was neutered before 6 mos of age.

That said, it is certainly possible that your cat wasn't completely neutered. He may have had an undescended testicle that the vet didn't remove, or the vet may have missed a bit of testicular tissue. In any event, a trip to a vet is warranted to investigate the source of your cat's behavior.

Laurie