Two things: First, usually topical formulas for transdermal delivery have to be tailored to the materials and so when a material is unfamiliar to me for this use (as T3 is) then there's no immediate answer short of going and working at it, and second topical formulations, or particular supplement formulations, I save for commercial products or potential commercial products and so while your question is reasonable, I can't put too much into an answer.
I would be reluctant to use T3 topically as there would be a tremendous differential in level the cells of the viable epidermis were exposed to versus the level achieved in subcutaneous interstitial fluid.
Also with all this, it's worth keeping in mind that for the most part penetration is only a few millimeters, basically within the skin itself. There is some dermal fat so that's not necessarily irrelevant, but when having many pounds of fat to lose, those pounds of fat are supplied principally by systemic circulation rather than local effect from topical delivery immediately above them. Interestingly, much of the "local" effect of topical delivery is really from systemic delivery. For example, topical delivery of NSAIDs to relieve joint pain can work quite well, with high concentration of NSAID in synovial fluid in the joint even though it is rather deep under the skin with all kinds of things that should carry the NSAID away. However, that mystery is solved as NSAID concentration in the synovial fluid of the opposite joint is equally high. Point being, to get efficacy at depth, you need systemic efficacy, but this will tend to achieve extremely high concentrations in the skin at the site of application. Sometimes this is fine, as with the NSAIDs and yohimbine, and sometimes not. I don't know with T3.