How much T4 thyroid is equivalent to an amount of T3? E.g. will x T4 generally be created to 1/2x T3?
This is the best answer I could find (from a page with more discussion):
In a mythical "normal" human, around one third of the T4 is converted to T3. So you could argue that T3 is "three times as strong as T4".
When taking tablets, for example, changing from T4 only to mixed T3 and T4, it is common to estimate on the basis of T4 being one quarter the "strength" of T3.
So, 100 mcg T4 (thyroxine) could be similar to 25 mcg of T3 (liothyronine) OR 50 mcg of T4 plus 12.5 mcg of T3.
This seems a reasonable basis for estimating doses but it is no better than an informed guess.
BUT if you are NOT converting properly this falls down. Imagine if you are taking 100 mcg of thyroxine and NONE of it is converting. So you have NO T3. You then take just 10 mcg of T3 and that has an effect which the T4 NEVER had. So there really is no way you can think of one being stronger than the other. One does something; the other does not