Log in
Thyroid UK
91,808 members106,070 posts

T4 to T3 Conversion issues? Is B12 the issue?

I have had secondary hypothyroidism since age 17. After lots of bad endocrinologist experiences (not referred to endo when FT4 38 and FT3 1.6, put on thyroxine, then taken off by ends who didn't care how unwell I felt.... it goes on and on!!) and miss diagnosis it has been a working diagnosis that I have a hypothalamic pituitary axis issue suppressing my TSH response.

I have been treated on Levothyroxine since diagnosis (now 39) and currently taking 225mcg per day. Until recently taking 250mcg and have maintained normal range FT3 and FT4 but October 2017 suddenly FT4 shot up to 32 (T3 not done as moved into new area) reduced dose by 25mcg and restested 6 weeks later, FT4 31 and FT3 4.6.

GP refused to check antibodies and vit d but have has ferritin, folate, B12, FBC, Bone, LFT, CRP..... all "normal" but I can see that B12 and Haemoglobin is just above the lowest end of range.

Feel weird, hypo symptoms. Dizzy, disconnected, tired, worried about having the energy to get me though the day, aching muscles, nerve twinges and tingling sensation in feet, cold, weight gain, abdominal discomfort.....

Serum free T3 4.6 (3.1 -6.8)

Serum free T4 31 (12.0 - 22.0)

TSH 0.014 (0.27 - 4.2)

Serum vitamin B12 level 198 ng/L [180.0 - 866.0]

Serum ferritin level 57.9 ug/L [13.0 - 150.0]

Serum folate level 9.8 ng/mL [3.9 - 26.8]Full blood count


Haemoglobin concentration 126 g/L [120.0 - 160.0]

Thanks in advance

4 Replies

Yes, you certainly do have a conversion issue, there. And, your B12 is so low that your doctor ought to be testing your for Pernicious Anemia. Of course, your B12 being so low won't help with your conversion problem, but it's not that alone that is causing the problem.

Many things can cause poor conversion, such as low calorie diets, or high antibodies, but sometimes you can never find the cause. So, it would be better if you could lower your levo dose, and add in some T3, to make you feel better. But, you do need to sort out your B12 because a lot of your symptoms will be down to that. :)


You didn't include your "normal" CRP results. Best to post all your recent results WITH the ref. ranges. Many medics incorrectly refer to elevated CRP as "normal".


Essential to test vitamin D too



Ever had thyroid antibodies tested?

Typical post where low TSH, high FT4 and low FT3 are due to terrible vitamin deficiencies




See Box 1. Towards end of article

Some possible causes of persistent symptoms in euthyroid patients on L-T4

You will see low vitamin D, folate, ferritin and B12 listed


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:

Prof Toft - article just published now saying T3 is likely essential for many. Looks like you are one of them


You might consider DIO2 Gene test too



Factors such as thyroid receptor numbers and their ability to function properly can play an important role in thyroid function. Cortisol levels need to be within normal range (morning saliva level of 3.7 to 9.5 ng/mL) and vitamin D levels need to be between 50 to 70 ng/mL for thyroid receptors to respond properly. Iron also plays an important role in thyroid hormone synthesis. Thyroid peroxidase activity depends on iron; therefore, iron deficiency could lead to hypothyroidism. Ferritin levels may need to be in the range of 90-110 to achieve proper thyroid function [Source: Paoletti]. health.howstuffworks.com/hu...


You may also like...