I've used up to 300mcg. Believe it or not this was during a "bulk" for about 3-4 weeks but I had ramped up from 100-200mcg the several weeks prior. Really it was just a reckless blast during a breakup but I shoveled in the calories and I had incredibly full muscles, bursting at the seems full with no bloat. Fantastic workouts. They were not clean calories, im talking 2-3 frozen pizzas in the evening followed by a pint of ice cream or two, all after a your standard big ass bowl of chicken/rice. I of course was on a... hefty amount of AAS.
Side effects were terrible, high BP, heart rate after eating whilst laying down of over 130+. I know of others online that pushed well over 200mcg too. I would never dream of doing it again without being on a beta blocker but even then I think 150mcg is more then enough to get whatever job done you are wanting to get done with T3.
37.5mcg puts me between 4.0 to 5.0 TSH (slow but in range). 50mcg year round has me around 2.0 TSH. One size does not fit all when it comes to the thyroid.
75mcg on blasts whether growing or cutting is my standard, I have only ever seen positive effects from this dose when attempting to gain. My 75mcg might be your 50mcg though, we are all different when it comes to thyroid hormone response.
Edit - Strong first post I know. I have been out of the game for 2 years, life happens. On my way back in now, previously went under board name Sector on other forums and no I am no where near that reckless anymore, I would like to see the age of at least 60.