Thyroid UK
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Relative values

Fed up wth my Dr I spoke to my highly respected pharmacist in my large village whose reputation for having an excellent pharmacy has existed for a very long time .

Out of interest he said that the generally accepted levels in one grain of Armour are 38mcg of T4 and 9mcg of T3 but there were definitely slight changes in proportions from time to time and the other interesting fact he gave me ,having been meticulous in his checking is that T3 is FOUR times as potent as T4.

I do not want to cause offence but we are all told on this site that it is THREE and so with great respect I do wonder whether there has been a change of scientific fact on this knowledge?

With thanks for all past help and information.

6 Replies

It is a ridiculous comparison. T4 is inactive. T3 is active. So if someone takes T4 only and they don't convert it into T3 in the body, then how does T3 compare to T4? It doesn't.

Petrol only provides energy when it combusts. A bottle of petrol is only a potential fire. It needs a spark to ignite it and oxygen to help it burn.

Thyroid hormones are the same way. Levothyroxine comparisons to Liothyronine are fraught with inaccuracies in comparison because Levothryoxine does nothing unless it is converted to T3.

If someone were converting Levothyroxine perfectly to T3 in the body, then the discussion about Liothyronine strength comparison is moot.

I hope this makes sense.

Depending on which source you consult, T3 is anywhere from 2.5 to 5 times as 'potent' as T4. But if T4 has zero potency in a given individual, then how does one calculate or compare the 'potency' of T3 in that person?


Totally agree with you, gabkad. I've been saying this for years! I always object when people start talking about T4 = xT3. In an ideal world, maybe. But hypos live in a far from ideal world. Nothing works as it should in their bodies. So, who cares about what a so-called 'expert' thinks about the way it should be? We have to live with what we've got. I Don't even see the point of asking.


Some people say it is 3 or 4 or 5 times as potent, but you only take it if T4 isn't working for you. So what happens in a rat or in vitro isn't really relevant. Really it's only a guideline if you are trying to work out whether to reduce levo or not when introducing T3. Empirical evidence from people doing that seems to suggest 3x is a good rule of thumb. Your free T3 and free T4 and (lack of) symptoms are the best guide to the correct level for you.


Just out of interest my apparently distinguished Endo who heads the Endo Dept in a famous London Hosp. Has said that he would not put me on more than 50mcg of T3 as anything higher than that is dangerous and will cause serious heart and Bone problems in the future I have no reason to doubt him and will be very careful and follow his T3 instructions as I do not want to stir up trouble for the future. Thanks for all your responses .I am in the process of learning all I can to get better after my pretty horrendous 8 years.


Yet there are many reports of people being on far higher amounts of T3 only for many years without heart problems or osteoporosis (as long as FT3 is in range). Did he give you the research references for this so we can all see them? I'd have thought it would depend on the individual - smaller person, lower dose; real conversion problems, higher dose etc.


Pig4u, I was prescribed 60mcg T3 after thyroidectomy.

Pig4u, T3:T4 ratios have varied between 15:1 down more recently to 3:1.

NDT manufacturers claim slightly over 4:1 ratio in their products.

It's ridiculous to state a 50mcg limit, people need what they need. I was prescribed 60mcg T3 after thyroidectomy. It's not 100mcg or 300mcg T3 which causes heart and bone problems, it's being overmedicated with FT4 and FT3 considerably over range for a long time.

The recently published Rotterdam Study found no association with TSH and atrial fibrillation, but found increased incidence of atrial fibrillation in unmedicated patients with FT4 high in range compared with those with FT4 low in range. It suggests research needs to be done to see whether similar results are found in patients on thyroid replacement.

Published recently in Clinical Thyroidology a meta-analysis of patients with TSH <0.1 found one excess hip fracture per 1,000 patient-years. I don't see this as significant increased risk. The article is behind a paywall.


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