Pernicious Anaemia Society
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Relationship between Thyroid levels and B12

I have been on levothyroxine for many years, at one time up to 125 mg, but after losing a significant amount of weight this was reduced to 75 mg. approx. 2 years ago I was diagnosed with B12 deficiency, and seemed to have been generally unwell since then, i am currently on 10 weekly B12 injections. 12 months ago after a routine blood test, I was told the levothyroxine dose was to high, and it was reduced to 50mg, a follow up test 6 weeks later showed this was to low and went back on 75 mg, next follow up test sowed this was too high and reduced to 50mg, this saga went on for a while until one GP set dosage as 62.5 mg. During all this time I developed a severe fatigue problem, where the simplest task left me exhausted and breathless, and continually falling asleep. after seeing another GP she ordered a full range of blood tests which all came back normal including B12, Thyroid tests, and Folate. But she then said the problem could be the levels of Thyroid and B12 could be "out of sync" and it was sometimes "difficult to get the levels correct in relation to each other", I am now waiting for an appointment to see a specialist in auto-immune diseases, as I am now developing signs of psoriasis. Has anyone else come across this, as when i questioned if too high a dosage of levothyroxine would cause further problems, she was rather dismissive and said try it and see if i felt better, in fact the fatigue problem has improved a lot over the last couple of months taking the higher dose of levothyroxine, but having spent years been told it as important to have regular checks as taking too much thyroid medication can cause serious health issues, I can unsure whether I should be following her advice as it does sound on here some GP's are taking pot luck on what works with these deficiency problems

8 Replies


I'm not surprised you are so unwell when your thyroxine dose is being changed so often. I too have hypothyroidism and b12 deficiency, getting a GP to increase your dose when your tsh is in range, which in GP speak is 'normal' , is a tough job and I was warned of dire consequences if I upped my dose. Eventually I upped it myself and felt very much better (tsh down to 2) so the GP agreed to prescribe the higher dose which was still only 75mcg !

You will find there are people with a lot of expertise on the Thyroid UK group on this site. If you can get your test results with ranges and post them on there you will get very good advice both on medication and on optimising your levels of vitamins and minerals to get the maximum benefit.

Best wishes.


a) what treatment are you on for B12 deficiency.

One possibility - particularly if your levels came back as 'normal' after a loading shot - is that you are being chronically under-treated.

b) there does seem to be a relationship between treatment with B12 and levo but yours would be the first time I have seen someone remarking that treatment with B12 meant they needed to raise their levo.

I believe both T3 (thyroid) and B12 are involved in particular processes that allow cells to release energy so a deficiency in one will mean that the process doesn't run as efficiently and correcting may affect the balance between the two - which would explain why people have reported needing to lower their T3 after starting on B12.

c) having low B12 implies that you have an absorption issue - and probably have low stomach acidity - as such this will affect how well you absorb levo - and one recommendation that I came across early after diagnosis was that taking levo with a something a little acidic - such as lime or lemon juice.

d) too high a dose of levo will result in the symptoms of hyperthyroidism which is dangerous, however, changes in dosage of levo take several weeks - and sometimes even months to kick in fully. If you are talking about increments of 12.5mcg - or even 25mcg then I'm pretty sure you would be well aware that you were overdosing well before it became a real issue.

In general using normal range as a single guide after treatment starts with levo is as problematic as using normal B12 range - most people seem to need TSH levels around 1 and T4 and T3 levels at the top of the range and in the case of T4 possibly over the range to really function properly post treatment.

There is a lot more information on TUK

e) Although it may seem counter-intuitive for a medic to take a 'try it and see' approach in relation to treatment, it's often the sign of a good doctor - there is so much variation from individual that at times medicine is more of an art than a science and having a GP who is open to seeing how a particularly individual does means they aren't trying to shoe-horn you into a one-size fits all approach.


Hi Gambit 62, After I had the loading doses approx.. 18 mths ago, I was put on a 12 week schedule for further shots, it was shortly after this various GP's decided I was on the wrong dose of Levo (it is difficult to see same GP every visit at my surgery) When I did get to see the GP I an registered with she changed the schedule to 10 weeks and put the Levo back up to 75mg, and said to avoid any more tests for 6 months. This is the GP who has suggested seeing a specialist in auto-immune disorders, as he can do more in-depth tests and investigations than she can. I think, as the increased dose of Levo seems to have resolved some of the fatigue problems, it might be best to wait and get his opinion, But many thanks for your advice.


glad the increase has had a positive outcome.

the more you say about your GP,the more I feel they are a GP you can work with and trust


Doses of Levothyroxine should NOT be regulated on the TSH alone. it is a Pituitary hormone and tells the Thyroid to produce T4 - a storage hormone - which needs to convert into the Active hormone T3 - needed in every cell of the body. So the TSH is a small part of the story and of very little value once you are on Thyroid medication. The FT3 is the most important test and rarely done on the NHS. Private Testing in the home is available through Thyroid UK and well used by members - see link below ...

Have you had your anti-bodies tested to rule Hashimotos ? Psoraisis can be linked to gut health and with low thyroid/Hashimotos this happens a great deal that the gut is not good. I think it also could be auto-immune.

You will also need to know your levels of Ferritin - stored iron - as some of your symptoms could be due to low iron - like breathlessness and fatigue. Ferritin needs to be halfway in range for thyroid hormones to work well in the body - that is convert T4 into Active T3.

Do you know your VitD level ? - result needs to be around 100 if in the UK. Folate also needs to be mid range.

Never let anyone mess with your dose of T4 unless you have all the results in front of you. You are only over medicated when the FT3 is over range .... You are entitled to have copies of all test results - they are legally yours. It enables you to monitor your own health and to check what has been missed by your GP .

I am not a medic - just a Hashimotos gal - with a B12 issue - and Crohns diagnosed over 43 years ago - now well :-)

Taking too much T4 AND too little levo can cause health issues. From what I see on the Thyroid UK forum here on HU - where there are over 70,000 members - it is often too little Thyroid medication that leads to issues.

Happy to help ....


Question for you (if you dont mind answwring please) I too have Pernicuous Anemia, Hashimoto's and Have had 2 flare ups of Ulceratuve Colitis. I am currently in remission from the UC and have been for over 6 tears. Does this mean I wont have a reoccurrence?

Thank you,



So many things can cause a flare up - so very difficult for anyone to predict. Six years is a good length of time to remain good -so fingers crossed ....

It may be a good idea to join the Thyroid UK Forum here on HU where you will find lots of support for Hashimotos - I have been a member there for over sIx years.

You need to make sure you are adequately treated for your thyroid and always obtain copies of your test results so you can monitor your own health and spot what has been missed !!

Do you know your levels of Folate - Ferritin - VitD ? Often very low with Hashimotos. How long have you been diagnosed Hashi's and what is your dose ?

Will look out for you on the Thyroid Forum 😊 Happy to help ...


My sister also on levothyroxine and having B12 injections has a similar fatigue problem. On my advice she has given up wheat. I have 4 sisters and three of them are on thyroxine. My other sister has stomach issues and arthritis. I am not on any medication - but I avoid wheat, gluten, milk and it’s products, soya and all additives. I also take mineral and fish oil supplements. I am currently sporting a copper bangle to increase my haemoglobin as copper helps iron leave the iron stores. Red meat contains B12 and I eat that as often as possible. Every day if I could. I am sure because I have given up foods, it is the reason why I am not on levothyroxine.

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