I am having extreme fatigue and I did blood test and my TSH level was high (8.6)! but thyroid hormones are normal (t3 (liothyronine) T4)! what is causing fatigue?

Further testing. The fatigue may be still be related to the thyroid. You should have a full thyroid panel which would include total and free T4 and t3, (liothyronine) rt3 and fti as well as thyroid autoantibodies. There can be defects in conversion from inactive to active hormone or an autoimmune process may be active. If that is the case treatment may include some hormone replacement but also addressing the immune system.