Talked with a family doctor about this T3 / T4 usage when it's not needed to treat thyroid problems.
She mentioned couple things that are worth noting.
Using them will (obviously) increase your energy and mild increase can cause something similar to minimal mania. Basically you'll be more active and more productive in every aspect of your life.
While using them for extended periods of time can cause your natural levels to drop, she didn't deem using them even remotely as dangerous as many more commonly used substances for fat loss.
She warned however that one of the patients she knows actually went through a breakup simply because of the natural increase in their triiodothyronine levels (which will happen if you use T4) and the mental effects caused by it.. So using them on high dosages can have effect in your life quality.
tl:dr she didn't recommend using them (obviously, she's a doctor after all and we're talking about using meds that aren't necessary or needed) but didn't think it'd cause problems either if used wisely.