You do know that T3 (triiodothyronine) is a helluva lot stronger then T4 (thyroxine) right? You do know that by manipulating your thyroid gland, to increase your metabolism, you will need to increase your nutrition because of the fact that you will be sending your body into a exasperated metabolic state which could lead to catabolism? Remember, your not supplementing with a "thyroid-replacement" dose, your actually pushing a healthy gland (at least you would know because you would have blood work done before hand) past what it can do for 5-6 weeks.
Now with all the medical jargon and nonsense out of the way, is it worth it? If you get a legit product, stack it with some sort of anabolic agent and are willing to monitor your progress, then YES. However like others have said, their is always ECA.