Thyroid UK
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T4 & T3 potency comparison

Good morning luvs. first things first...... I want to thank everyone who gave me advise. Update!!!! Feeling a difference already with my new addition of T3. Still not consuming the whole 5 mcg yet, working my system up to it comfortably. Listening to MY body. So this is the question and please do not answer unless you know. I have hear everything from double to 10xs!!! How much stronger is T3 than T4???? I am guessing 4Xs possibly? I seen a chart once that said under synthetics which is me, Synthroid brand and Cytomel generic 25 MCG of cytomel was equal to 100 of Synthroid, but no comparison for the 5 of cytomel i was prescribed. And then you have to factor in the % of T3 you get that is coming from the synthroid correct? Our systems transfer 40-50% is it of T4 roughly into T3...... but that's if you convert and that's if you absorb properly also. Then there are people who convert but not properly or not enough lol! Wow is this thyroid stuff a mess! Like I said please only converse with me if you are able to answer these questions. Have a wonderful day.

2 Replies


Liothyronine (T3) is 3 x stronger than Levothyroxine (T4) so 5mcg T3 is equivalent to 15mcg T4.

T4 is a storage hormone and converts to T3. If TSH is low, FT4 high and FT3 low it indicates poor conversion.


I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.

1 like

thanks for the quick response. hopefully soon I will be able to work up to the full 5mcg. if i can fully adjust to that i will see if dr will raise my T4 dose a little. She dropped it gain before giving me the T3.


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