Do we still need to take T4 when taking T3?

Hi,

I've been on 75 mcg Synthroid for about 2 months now. I've reached a plateau and my Dr. wants to slowly add in some T3. I'm not completely better and increasing the T4 beyond 75 mcg doesn't help my symptoms to improve any further. My question is, if your body just converts the T4 to T3 anyway, why do people even take T4 at all? Wouldn't it make more sense to just take the straight T3 and then not have to worry about converting it properly? Or is there a reason you need to take both?

Thanks, I am new to all this!

PS..I do take my Iodine every day along with adrenal supplements..