T4 has to be converted to T3 by the body before it can be used and it is only 1/3 as strong as T3.
That being said, unless you're hypothyroid or a professional BB'er the risk of using T3 cannot be justified. There is the potential to burn out your thyroid forever.
I wish this was true as I do think people mis-use and abuse thryoid drugs, but as I have recently been diagnosed as being hypothyroid, I have expanded my knowledge about thyroid medication.
Most people who are hypothyroid are controlled with thyroxine
(T4) and the idea of getting the dose correct is that it doesn't fully suppress the thyroid gland, so you will still release TSH and your own thyroid hormones.
There have been cases where people have been put on thyroxine by accident, and they do return to normal thyroid function.
I think if you have pre-existing condition, you could precipitate a thyroid problem.
There is also an issue with the dose that people are taking.
Typically, the average dose for thyroxine is around 100 mcg/day, but it is slowly built up to this point, and if people require T3, I think it is a maximum dose of 20 mcg/day.