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Thyroid UK
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Is there a T3 to T4 ratio

Hello, I'm post thyroidectomy for papillary thyroid cancer and on Levo only. I always get my FTF blood results in a letter, sadly without ranges, and want to make sense of them. I know that my T4 has to convert to T3, so is there an optimum ratio? I'm interested in working out if I'm converting adequately.

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You have to make sure that the unit of measurement for FT4 and FT3 are the same. If they are the FT4:FT3 ratio for good conversion is generally between 3:1 and 4:1

Another way of knowing if you convert well is to get the ranges, if FT4 is high in range and FT3 low in range then you're not converting well.


Thank you

When my hospital did my last blood test I was FT4 30.2 & FT3 5.8 so that's provisional, as long as all units are the same and I think they are, a ration of 5.2:1 ?

The hospital then suggested I reduce my Levo from 150/125mcg to 125mcg daily and asked my GP to do another blood test (so I didn't have to travel to the hospital). Surprise surprise he (GP) wouldn't do the FT3

I'm back at the hospital in October and they'll do a full panel.

Thanks for your help.



It does suggest poor conversion and overmedication. If you're in the UK then generally the FT4 and FT3 units of measurement tend to be the same.

The other thing that can cause high results is thyroid antibodies - have these been checked - Thyroid Peroxidase and Thyroglobulin.

Not surprised FT3 wasn't done at GP level, particularly if TSH was within range. What were your results on the reduced dose of Levo?


Yes, I've had thyroglobulin antibodies detected and they waver. The hospital has never tested for thyroid peroxidase, as far as I'm aware. How do antibodies cause high results? I often have difficulty in making sense of this if I don't have a thyroid.

When done at the GP's my FT4 came down to 28.3 and TSH <0.02

I'm in the category of high risk of a reoccurrence so TSH needs to be <0.1



I'm not well up on thyroid cancer so can't speak from experience. If there are antibodies then they can fluctuate and this can cause fluctuations in results. Whether this can still happen after thyroidectomy I don't know.

Assuming your doctor knows that your TSH needs to be kept <0.1 then they are going by the high FT4 which is understandable at that level.


Thank you. I suspected my conversion was off. But this gives me more information and better vocabulary to discuss with my doctors at future appointments.

This is what I find confusing, sometimes, is that most of the information I find online is aimed at individuals who have an intact thyroid. So, I get that having these various antibodies will have an affect on the thyroid gland and change FT4 FT3. But working out what this all means for me post thyroidectomy is tricky.

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This is a very meaningful link to your inquiry.

I will also quote relevant excerpts:

"Clinicians noted several differences in the ability of l-thyroxine monotherapy to normalize markers of hypothyroidism at doses that normalized serum TSH (45). "

"...higher serum T4 levels will impair systemic T3 production via downregulation of a deiodinase pathway (9).

"In a large study of approximately 3800 healthy individuals (4), the serum free T4:free T3 ratio was around 3, as opposed to a ratio of 4 in more than 1800 patients who had undergone thyroidectomy and were receiving l-thyroxine monotherapy."

To avoid any additional quotations, I will refer you to the section entitled "The Future".

The takeaway, from this article, regarding your interest is, I think rsther ironically, that if you want to increase your conversion, you must lower your t4 dose, as SeasideSusie seems to suggest.

In fact, that "...higher serum t4 levels impair systemic t3 production..." should cause a shockwave throughout the community of patients undergoing t4 monotherapy! It certainly explains the all too common phenomenon of continual hypothyroid symptoms, even worsening symptoms, with t4 dose increases.

I think it's called a paradox: to have more conversion and less of the symptoms associated with low serum t3 levels, we must take less t4.

I hope this helps.😊


That does help. I'm actually open to them reducing my dosage. I can see I am very suppressed and my FT4 is high. I'm open to them tweaking it till I don't feel so sluggish.

I did get dropped down by 25mcg, to 125mcg daily, and I was still very suppressed and FT4 was still over the top range.

I 'd always imagined I'd need much more with not having a thyroid, but I can definitely see myself on 100mcg at some point.

Thank you once again.

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For full Thyroid evaluation you need TSH, FT4, TT4, FT3 plus TPO and TG thyroid antibodies and also very important to test vitamin D, folate, ferritin and B12

Private tests are available. Thousands on here forced to do this as NHS often refuses to test FT3 or antibodies


Medichecks Thyroid plus ultra vitamin or Blue Horizon Thyroid plus eleven are the most popular choice. DIY finger prick test or option to pay extra for private blood draw. Both companies often have money off offers.

All thyroid tests should ideally be done as early as possible in morning and fasting. When on Levothyroxine, don't take in the 24 hours prior to test, delay and take straight after. This gives highest TSH, lowest FT4 and most consistent results. (Patient to patient tip, GP will be unaware)


Thank you, I got the majority of the blood tests that you've listed done at the hospital (Northern Centre for Cancer Care, Newcastle, UK). It was just this one occasion that they suggested I call into the GP for a quick blood test, after reducing my levo, rather than making an extra journey to the hospital. My GP still flips at seeing my TSH but doesn't understand the role of TSH suppression in thyroid cancer.

I'm on 125mcg of levo and no thyroid. I'm pretty much over-medicated to keep my TSH less than 0.1 but that means my FT4 is often high. My overall concern is converting to T3

Thanks for your information.


Dr Toft, past president of the British Thyroid Association and leading endocrinologist, states in Pulse Magazine,

"The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range - 0.2-0.5mU/l.

In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l.

Most patients will feel well in that circumstance. But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.

This 'exogenous subclinical hyperthyroidism' is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l)."

You can obtain a copy of the articles from Thyroid UK email print it and highlight question 6 to show your doctor

 please email Dionne at


Professor Toft recent article saying, T3 may be necessary for many otherwise we need high FT4 and suppressed TSH in order to have high enough FT3. Note especially his comments on current inadequate treatment following thyroidectomy or RAI


When your FT4 was high at 30 FT3 was 5.8

Can you add ranges on those results

It suggests poor conversion

Many of us need FT3 above 6

What are your actual vitamin results ?


I'm only given the results, my blood tests are done in the UK by my hospital

On paper, the FT3 (3.5-7.8) range suggests I should be fine at 5.8?

I feel "okay" but feel I should be a bit more than okay (if that makes sense).

Would poor conversion be a zinc, selenium and magnesium deficiency?

And therefore this is more of an issue to get the best out of the T4 i'm taking?

Or am I missing the issue?


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