Stubbornly elevated ALT and AST

Hello everyone. Initially I have posted my question in the digestive health group, but I was referred here, so I hope that I can get some answers...

What could be causing the constant elevated numbers?
ALT in particular has been in the triple digits over the past few months, AST varies all over the place but it is again over the reference range.

For reference, I don't drink, I don't do any recreational drugs and I eat a healthy diet. I am really into physical activity as well. I don't have a problem with obesity, in fact, I am underweight. I don't take NSAID drugs or any other drug that is notorious for liver health

My doctor has done a series of tests in order to exclude any possible issues with my liver, such as ultrasound (no issues found i.e. I DO NOT have a fatty liver), tests for Hepatitis A, B and C (all negative). No issues with my gallbladder or any other part of my digestive system. My thyroid is functioning well.

Is there anything "hidden" that could be causing the stubbornly elevated ALT and AST? I'm particularly worried about the ALT, because it has been rising over time.