It really comes down to personal preference. T4 is converted into T3 in the liver, and many people cannot make this conversion properly, so it is better they take T3. T3 should be taken multiple times a day, in smaller doses, under 6.5mcg per dose, where T4 can be taken once per day and that is an easier schedule for most people. That's why doctors usually prescribe only T4 and not T3: it is easier to take one pill a day. But doctors are tragically uneducated when it comes to thyroid medication, and they ignore the population of people who simply can't convert T4 into T3 properly, so most people end up taking T4 and don't feel enough of an improvement.
Some people like T4 and T3 together, and feel better that way. It's highly individual. T3 is good because basically everyone benefits from taking tiny amounts of T3, and people can dose it in a very flexible way. Did you exercise too hard and shut down your thyroid? Take a little T3 and your thyroid comes back. Do you need to digest a super heavy meal and want to increase metabolic rate to make this happen more quickly? Take some T3. So there are examples where you can take small amounts of T3 but not having to be committed to taking as a thyroid replacement therapy, but as an occasional thyroid enhancement.
For fat burning, using T3 is preferred but I don't find it to be a great fat burner. There are better options than T3 for that, but where thyroid medication shines, is that it keeps your metabolism functioning optimally during a cut, when calories are low and workouts are increasing.
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