30-40mcg of t4 is a reasonable dosage. RX tabs are 25, 50 and 100. Most ppl get 25 or 50's.
T3 comes in 2.5mcg & 5mcg.
Some believe t3 can be used to "jump start" the thyroid, but if you're somewhat or straight up hypo then I'd say see if iodine can get it into the mid range.
If not, go see an endo, tell them you're gaining fat and feel like you have no energy, and believe thyroid to be the cause. Most will Want the thyroid in mid to upper range. So they'd prolly rx you 25 -50mcg/day (t4) depending on how low, and the endo. Some use t3 if it's really low or if your body doesn't convert t4 well.
I'm sure I'll get flamed and ppl will say use much higher doses, but this is what the Medical community will do. And it usually works. If you wanna do it yourself, 25 -50mcg of t4 (usually less suppressive than t3) should be fine, depending on how low you are. Some numbers from bloodwork would help here.
Not medical advice*
Best of luck.
