Is it typical for liver function tests to fluctuate significantly?

Which ones? If you have a benign condition called gilbert's syndrome, bilirubin rises in times of stress and normalizes when the stress is gone. If your aminotransferases fluctuate it could be related to alcohol, drugs, hepatitis, or other conditions. If your alkaline phosphatase and bilirubin fluctuate it could your gallbladder. Talk with your doctor or a liver specialist.
No. No and usually related to drug reaction (and yes body building and weight loss supplements are drugs) .. .. Usually gets better by stopping the offending drug ..... Also viral infections can cause a spike and then trend down.