male cat aggression towards other cats

Question
My two cats (one male about six and one female about months) and I have recently moved in with my sister (who also has two cats, both female.) My male cat will not get along with her two cats. We have tried to keep them separated for awhile and then put them together, but he chases them and attacks them. We thought it may be because her younger female cat was in heat, but she has been fixed since then and he is still mean. I have researched this problem and some people say it may be a thyroid problem..but he has never acted up like this before. Recently I was not home and he was chasing one of the cats and my sister yelled at him, and he turned around and attacked her. He has never hurt me or my other cat but I am at a loss of what to do. He has to stay locked up in a small room most of the time and I just feel terrible for him. He is a good cat, I just don't know how to get him to stop being mean to her two cats? Please help.

Answer
If her cat has only recently been fixed, then that could still be the reason why your male cat is acting so aggressively.  The hormones that are released into the body during the heat cycle can sometimes take up to sixth months to completely leave the body.  One of my own male cats had this same problem.  I couldn't figure out exactly why he kept attacking my other cats (he even got the nickname "Angry Cat").  Then I realized one of my females had gone into heat very early and he had actually started sensing it before she starting it showing it with her behavior.  I had her fixed immediately but he continued to attack my other cats for quite some time.  There is a product on the market called Feliway that you can also try.  It comes as a diffuser that you can plug into your electrical outlet that releases a pheromone that is chemically similar to the one that nursing queens have.  Provided your male cat was nursed and not hand reared, they have had some success with this helping to calm down aggressive cats.  It can take up to a month to work however.  Sometimes cats can become aggressive when they are not feeling well, so you may want to take him in for a physical to rule out that possibility.  Finally, if you and your vet have ruled out anything physically being wrong, and the one female has been spayed for quite some time you may want to talk to your vet about anti-anxiety/anti-depressant medication such as Prozac or Amitriptyline.  These medications can help calm down cats that have become aggressive due to psychological problems.

Amanda