T4 or T3 thyroid hormone: which is better for weight loss? Well, if I’m being honest then T3 thyroid medication will win just about every single time. This is because T3 is the most biologically active thyroid hormone. In addition, it doesn’t require any activation by the body in order to be used by the body. This doesn’t mean you can’t lose weight if you are using T4 thyroid medications like levothyroxine or Synthroid. But it does mean that these medications will generally only help you lose we