Elevated ALT and AST

I am a 30 year old female, do not drink or take drugs.  I gave blood in June and found out I had elevated levels of ALT , 106. In Jan I had given blood but all was fine.  

Since then I have been through a series of doctors and now I am with a specialist who has done dozens of blood test.  I completed and ultrasound which came back clear as well as my cholesterol is at 133, HDL, 47 and LDL 72 and tryglycerdies at 70.  My ferritin is at 20.  Iron is normal.  Now my ALT is 424 and my AST is 284.  Heapatitis tests have been done by three doctors and all are negative.  What could this be??  I have been told since the cholesterol and the ultrasound and the triglycerides are alll normal, it is probably not a fatty liver??  I am overwieght,  I am 5'4" and 175 pounds.  I also have been told that with how high it is and how quickly it has risen we need to do a biopsy.  Would the levels rise this high if it were a fatty liver and continously rise??I do have an overactive thyroid that they just found but was told that it has nothing to do with the liver as it would have to be much higher that it came in at.  (T-3 uptake,22),(T-4 free, 1.1) (free T-4 (T7), 2.9) (thyroid simulating .12) and thyroid total is 13.

Sorry so lenghty but I wanted to give you as much information as possible.

Please help!