Thyroid UK
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Brain Fog. Is this directly linked to the condition hypothyroid or T4 replacement? Would adding T3 help?

I am really struggling with the simplest of tasks that involve using my brain. Lots of silly little mistakes and very poor memory. I am currently on 125mg of levothyroxine. Due new blood tests next week. Also suffering with depression, burning feet syndrome, low temperature and low pulse rate and the usual aches and pains. My last bloods were as follows:

TSH - <0.01 (0.35 - 4.94)

T4 - 15.9 (9.0 - 19.1)

T3 - 4.07 (3.6 - 6.5)

B12 - 311 (187 - 883)

Vit D - 36

Folate - 9.4 (1.8 - 18.3)

Ferritin - 47.1 (10.0 - 204.0)

Doctor advised to take vit D which I am doing.

Worried that if this continues I will not be able to find another job. I have just resigned from my teaching job as unable to cope.

Any advice would be greatly appreciated.

Many thanks


6 Replies

Both your B12 and ferritin (iron store) levels are low and both can cause brain fog, as can the low VitD. I would be concentrating on getting these levels up (preferable with your GP's knowledge and help) before thinking of T3 as your TSH is low.

You should be aiming to get your B12 over 700 (you cannot overdose on B12 as any access will just flush through your system) and your ferritin nearer 90. Your T3 is also low but with your B12, VitD and iron low then your body could be having trouble converting your T4 into T3. How much vitD are you supplementing? - taking 1,000iu daily will maintain your levels and anything over will increase your levels.

Hope this has helped.

Moggie x


Hi Moggie

Thanks for your advice. I was until a few weeks ago taking 25ug vit D, but changed to 5000iu after reading advice of the TUK forum. I am also taking 1000mcg of Methyl B-12. What is slightly worrying is that since I started taking the B-12 I started having the burning feet. Do you think I need to take more?



I'm sorry but I don't know - what I do know is that feet issues can be a hypo symptom. Try googling "soles of feet + burning sensation" or "low B12 + burning feet" and see what you get. Google is a great tool for research.

Moggie x


Hi, some people experience an initial worsening of symptoms when they start B12, believed to be the nerve endings "waking up" again or something like that! I recently had my loading dose (6 hydroxo injections over 2 weeks), and I had a surge of nerve activity at first, couldn't lie on my right side or back for a few days. I also had a constant headache and a couple of migraines up until about the 4th injection. It's about a week after my 6th injection and those symptoms have passed and I feel much better. I have started on the Methyl B12 spray daily and a B-complex to keep my levels up (I also take folic acid prescribed by the doc as my folate level was very low). You also need to keep your potassium levels up, have you ever had that checked? I try and eat lots of potassium rich foods. I would say you could definitely take a higher B12 dose (you pee out what you don't need, it's not toxic). For advice on this I would recommend posting a question on the healthunlocked community for pernicious anaemia, and you'll find links to other websites over there. My serum B12 level was similar to yours but I had the MMA test done privately which was elevated indicating that I did have a deficiency. That's how I got the injections, I was previously told by 3 other doctors that my levels were normal.


Hi Hampster1

Thank you for your advice. No I have never had my potassium levels checked, but will do next time I have my bloods done. Sorry if I am being a bit dumb (brain fog) but what is MMA?




Someone else posted this the other day, it's a really good summary of current thinking in this area, and explains active B12 and MMA:

The tests you ideally want to check B12 status are Active B12, MMA or Homocysteine. At present you can only get these done privately in the UK, I've seem other people on the forum doing an Active B12 test by post. My Endo arranged my MMA test for me, out of my private medical insurance. Do you see an Endo? GP's tend to be clueless about this. If you're going to have any of these tests you'll have to state that you've been supplementing, because ideally you'll need to stop the supplements to get a true result.

By the way Jarrow's do a 5000mcg methyl-B12 sublingual available from But if you're thinking of having the tests done don't take them yet!