I fried my thyroid gland about three years ago. I am not sure what did it but I know it was not T3 or T4.
I must take synthroid every day, for the rest of my life, and it sucks. Before I was diagnosed with a thyroid disorder I suffered from cold feet, depression, lack of mental alertness etc. The depression alone was pure fucking hell. I know others who have fried their thyroid glands with T3 and I wouldn't wish that upon my worst enemy.
Yes, T3 will help you lose weight. If you are the 10-20% of people though who end up permanently using T3 then it is definitely not worth it.
The thyroid gland regulates so much in the body that IMO it is not worth using it as a drug to lose 5-10lbs that can come off with exercise and diet in a matter of weeks.