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Thyroid UK
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FT4 increase after taking T4


is there a approximate guess by how much blood FT4 levels increase after taking T4/at how much percent they peak ?

I took my T4 before blood test by mistake but want to decide if I add in T3 or increase T4. Steel cold hands and low body temperature. Doc says all ok :( Labs are:

FT4 21 (11-25)

FT3 3.67 (3.01-6.16)

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You would reduce by 20% to get a rough estimate. Takes your FT4 to just below mid-range.

But your doctor doesn't know what he's talking about. Your FT3 is not OK, it is dragging along at the bottom of the range, when most people need it up near the top of the range to feel well. You could try one more increase of 25 mcg levo, see what that does. And, if that doesn't quite cut it, add in some T3.


thank you ! Is it ok to increase T4 dose according to a blood test where I took my T4 before ?


The ratio of your FT4/FT3 is 5.7/1. In health with normal thyroid the ratio is around 3-3.5/1. This is a clear indication that you are a poor converter of T4 to the essential hormone T3. This is why your treatment is inadequate. You have the classic signs of FT4 almost too high but FT3 not properly responding. I'd say you were a good candidate for combined T4/T3 direct therapy. A rT3 test would show that. It would give a high reverseT3 result because your T4-T3 body conversion system is working as hard as it can but still cannot raise your FT3 to good levels. The high rT3 would demonstrate your body is getting rid of the excess T4 which can't be usefully used.

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I took my T4 before the test so FT4 would be a bit lower as would the FT4/FT3 ratio be. But my FT3 is too low anyway. Since the doc won’t prescribe T3 I could order online. How much should I start with ?


Do you have Hashimoto's also called autoimmune thyroid disease diagnosed by high thyroid antibodies?

If so low vitamin levels are extremely common, have you had vitamin D, folate, ferritin and B12 tested? If so add results and ranges

What supplements do you take?

If you have Hashimoto's, Are you on strictly gluten free diet?

Your FT3 is low, this can be because you are poor converter because of Hashimoto's, and/or due to low vitamins

Gluten free diet can improve gut function and vitamins


Complete opposite. All vitamins high in range. Just ferritin at the low end.


Ferritin needs to be at least half way in range for thyroid hormones to work. How low is it

If you have Hashimoto's, then likely need to be strictly gluten free before consider trying a small dose of T3

Ideally ask GP for coeliac blood test first








at 45 (30-400). Thyroid antibodies all negative so I guess no hashimoto's.


Not necessarily, apparently 20% with Hashimoto's never have raised antibodies


An ultrasound may be useful. Have you had one?

Have you had both TPO and TG antibodies tested or only TPO. Some patients only have raised TG antibodies, but NHS won't test TG unless TPO is raised


Have you had full iron panel to check for Anaemia as ferritin is low?

Eating liver or liver pate once a should help improve ferritin. It needs to be at least above 70 for good thyroid function


Had couple of ultrasounds done which showed nothing wrong, all good. Iron labs were: Iron 18 (11-29); Transferrin 2.77 (2-3.6); Transferrin-saturation 25 (16-45); Ferritin 45 (30-400).

I had done TRAK and TPO antibodies.

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So you are a poor converter with no obvious cause

Either need more Levo or addition of small dose of T3

Prof Toft says many are poor converters

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


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yep I think I might just need more T4 too since my FT4 is just mid-range. Guess I'll increase by another 25mcg.

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