elevated liver enzymes

Just hoping to get some information...I recently had some blood work done which revealed elevated ALT enzymes (120). I also had two abdominal ultrasounds and CT scan of my of my abdomen which was normal except for a gallbladder polyp. I had a blood test for viral hepatitis A,B,C which also was negative. I met with a gastroenterologist and he seemed concerned about the liver enzymes, so much so that he ordered more specialized liver panels. My question is what can cause the liver enzymes to be elevated in the light of two negative ultrasounds and a ct scan? He did mention some other types of hepatitis such as autoimmune or alcohol induced. Any input would greatly be appreciated. BTW, I'm a 38 year old otherwise healthy male. Thanks.