First off, messing with your thyroid hormone levels could have long lasting effects. Having said this, some informed users have reported some success with using T-3 for fatloss (anecdotally). If you do not know what you are doing, or have no clue as to where your current values are, you have no buisness using this stuff. Please also remember I am by no means an expert on this subject, but have done quite a bit of research on the subject.
Enough with the safety stuff for now. . .
There is much more to thyroid than is discussed on the boards, or in the community as a whole. While this may be old news to some, there are also T-2s to think about as well (along with T-1s and T-0). There is no current literature to be found (by me at least) on T-1s or T-0s in pubmed. I have, however found a couple of interesting studies on the T-2s (AKA 3,3 diiodolthyronine and 3,5 diiodolthyronine). One of these papers is titled "How the thyroid controls metabolism in the rat: different roles for triiodothyronine and diiodothyronines", Journal of Physiology (1997), 505.2, pp.529-538. And also "Calorigenic effect of diiodothyronines in the rat" Journal of physiology (1996), 494.3, pp.831-837. Both of these studies reported effects of the T-2s on different tissues and resting metabolic rate. What I have gleaned from this is that the both of the T-2s increase metabolic rate in specific tissues of the body (specifically brown adapose tissue AKA BAT, and the liver). These also seem to have very little effect on cardiac muscle (where T-3 is speculated to have an effect here). The studies also show more effects for a higher dose (dose dependant), and quicker onset of increased metabolism (24hr onset vs up to 72hrs for T-3). There are a few things to mention about these studies though. #1 they were conducted on rats only. #2 the T-2s were injected IP. #3 there was no actual measure of fatloss whatsoever. So please take these for what they are.
On a similar note, there are products on the market containing T-2s as a primary ingredient (OTC supplements). I have even heard that Biotest has at one point dipped its toes into this pool. any truth to this?