FT4 to FT3 conversion with low BMI

[Caroline888]

jimh111

Hi Jim,

I am interested in the following statement you made when replying to Peroxideblader's post earlier today:

"People with a low BMI have a little less T4 to T3 conversion than people with a high BMI."

I have always had a low BMI (19 - i.e. at the bottom of the normal range) and although my FT4 level is towards the top of the range on 75 mcg levothyroxine, my FT3 has so far only measured barely halfway through the range. Are you saying that this is normal and to be expected with someone having a low BMI, or are you saying that a person having a low BMI will struggle to convert FT4 to FT3?

I do feel quite well at present with most of my symptoms having resolved but would like to see my FT3 at a higher level since it is regularly stated in the forum that it should be at least halfway and preferably towards the top of the range.

I would be so grateful if you would expound on that statement.

Best regards

Caroline

[jimh111]

Caroline, It's usual to have a mid-interval fT3 with high normal fT4 for patients on levothyroxine. Most patients with primary hypothyroidism (diagnosed with high TSH) do well in this situation although some need some T3. The statements that fT3 should be above mid-interval are generally incorrect, at least for the majority of patients with primary hypothyroidism. Many patients who are doing well do not bother with thyroid fora, so we get bias when looking at how patients on this forum do. Some patients do need a high normal fT3 or even quite high fT3 levels, but many do not. If you are doing well don't target a higher fT3. Clinical response is much more important than the numbers, both ways higher or lower.

Patients with a higher BMI tend to convert a little (not much) more T4 to T3 having slightly higher fT3 and fT3/fT4 ratio. It's not a case of them being unable to convert, it is probably a normal adjustment to body mass. I wouldn't stuggle to alter my BMI for conversion purposes. If you are interested in the technical details here are a couple of papers, I read the first one some time ago but haven't read the second.

researchgate.net/publicatio...

ncbi.nlm.nih.gov/pmc/articl...

[Caroline888 in reply to jimh111]

Thanks very much, Jim, for your prompt and reassuring reply and for the links which I will look at later this evening.

As you say it’s how we feel that matters the most and I have not worried unduly about my FT3 level. I certainly feel a lot happier about it though after reading your reply but will continue to try and improve conversion by optimising vitamin/mineral levels.

Thank you so much for taking the time to explain and advise. I’m grateful.

Best wishes

Caroline

[Angel_of_the_North in reply to jimh111]

Since BMI doesn't take body composition into account, do you think this is lack of muscle mass or lack of fat (or both)?

[jimh111 in reply to Angel_of_the_North]

I don't know! I think it's more a case of deiodinase increasing a touch in obesity rather than a lack of muscle or fat. Deiodinase takes place in muscle but on the other hand it could be an indirect effect of other hormones that vary with body composition. I think it is an academic issue as the effect is small and changing body shape is not easy for most people.