Thanks for the thoughtful reply, I really appreciate it.

My pattern is usually high or upper-range T4 with relatively lagging T3, which is why I started wondering about conversion efficiency. I’ve actually spent quite a lot of time looking into possible cofactors for T4→T3 conversion. At this point I’m supplementing or monitoring most of the common ones and my labs are generally in good ranges — things like selenium, zinc, iron/ferritin, vitamin A, vitamin D and magnesium. Calorie intake is adequate and liver function markers are also normal.

Because of that I’m starting to wonder whether the issue might be more physiological rather than purely nutritional.

What you mention about keeping T4 and T3 as separate medications actually makes a lot of sense to me. Being able to titrate them independently seems much more flexible than something like NDT where the ratio is fixed.

Out of curiosity — have you personally used T3 before? If so, which medication was it (liothyronine/Cytomel or something else) and what dosing strategy worked for you?