Synthetic T4 (Synthroid) will not work if the enzyme required to convert T4 to T3 is not normal. Often patients are placed on Synthroid or a similar drug for low thyroid and feel no different even though the TSH levels become normal. The reason is obvious. Their T4 is not converting adequately to the active T3 hormone or not enough T3 is getting into cells for whatever reason. The blood test looks great — the patient feels lousy.
You need to be very carefull with this. You could reduce the T-4 dose and add a small dose of T-3 to compensate for the differance (not mcg for mcg as T-3 is much more potent). Perhads drop the T-4 to 50mcg and add 10mcg T-3. Run that to access tolerance and slowly ramp dose in small incriments like 5mcg.
I am no expert, this is more theory than fact so take it as that.