TSH=0.04 (0.4-4.5mIU/L)FreeT4=1.5 (0.8-1.8ng/dL)FreeT3=2.5 (2.3-4.2pg/mL). Have Hashimotos disease, on 112mcg Synthroid. Why is tsh so low FT3/FT4 normal?

Any symptoms? All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Having said that, just one parameter, in this case TSH, slightly out of whack is not a cause for alarm. You may watch and wait. If your clinical situation changes it may warrant repeating the tests. Wish you good health!