Thyroid UK
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Test for active form of B12 versus inactive

Hi Everyone. I am new to the forum and very glad that I have found it. I have had health issues since becoming pregnant with my last child 24 years ago. It took many years for hypothyroidism to be diagnosed from a TSH test. Was told I was borderline for years. I have been taking Levothyroxine for 15 years (currently taking 100 mcg). My story is very similar to most people on here. One of poor diagnosis and even poorer treatment and the resultant lack of optimum health. I have managed to get myself reasonably well with very careful lifestyle management i.e diet and supplements, sleep and moderate exercise. However, I have decided that at my age (62) I can no longer accept not being properly diagnosed and treated accordingly as I am very aware of the long term damage done from poorly treated thyroid issues. I am currently 4 stone overweight and have little to no success regardless of my efforts to lose some of this excess. My most recent tests done on 9th of June this year are:

TSH 0.24 range 0.30 - 5.00

FT3 4 .00 3.20 - 6.80

FT4 21.00 12.00 - 22.00

B12 632 180 - 700

Cortisol 368 170 - 500

It looks to me as though I have a conversion issue as my FT4 is near the top of the range and my FT3 nearer the bottom. However my GP disagrees and says that everything is fine. I also have an issue with low body temperature which gets lower in the evening. I believe it gets dangerously low as it has gone down as low as 34.5 when the weather was very cold. At present I am usually around 35 - 35.5 in the evenings. I rarely reach the norm of 37. Again my GP was unconcerned but with badgering from me he has asked me to keep a record of temp 4 or 5 times a day for 5 or 6 weeks and he will view the results. I have just ordered the Thyroid Plus 12 test from Blue Horizon. I am now confused because it says that this tests for inactive B12. I queried this with them and they say that this test won't show the amount of active B12. Another Active B12 test is needed to get a better B12 result. They offer this test at a further cost of £39. Can anyone comment on this as I haven't seen this mentioned by anyone else although a lot of attention is given to B12 status but not whether it is Active or Inactive. I would also be very grateful for any comments on my test results above. I apologise for the length of my post and thank everyone for their input. I don't feel quite so alone or helpless now that I can see so many in the same situation.

4 Replies

The B12 test routinely done is the serum test which includes both the Active and Inactive B12. So when you receive your result you need to realise that only 20% approximately of the result is available to be utilised in the cells - where it is needed.

The Active Test indicates the amount that is actually available to be transported to the cells. Neither are perfect - but like most blood tests, are good to be used as a guideline.

The above link takes you to everything you need to know about B12 - lots of reading :-)



Welcome to our forum.

FT4 is good but FT3 (which is the active hormone so responsible for well being) is low, indicating you are not converting well. Feeling cold & weight gain are classic symptoms of low thyroid hormone.

Optimal Vit B12, Vit D, folate and ferritin will help with absorption and utilisation of thyroid meds. Supplementing 100-200mcg Selenium is known to aid thyroid hormone conversion.

A serum B12 measures the active (available for use) and inactive (unavailable for use) B12 in circulation. High serum B12 that can not be used properly by the cells will be exported back to the blood where it binds to haptocorrin.

This is then imported by the liver and recycled via bile to the gut (salvage and recycle pathway for B12). This B12 will be unusable to the bodies cells making you deficit in B12 with a high serum level reading.

Many Hashi sufferers have difficulties in absorbing & using Vit B12. The active (holotranscobalamin) B12 test (HoloTC) measures only the active.


Supplements Known To Aid Thyroid Health




Active B12 testing

1 like

radd ,surely missed "not" converting?

1 like

Oops .... Typing too fast & missed probably the most important word there ! ! ..

Have edited post to read "not" converting.

Thank you Treepie,


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