A question about ft4

[Rock_chick1]

Always trying to get my head around this and learn as much as possible. So here is my question. When we are on T3 and T4 medications. The T3 goes directly to the cells I believe, so why do some of us still need a good level of ft4 (the converter hormone)

[SeasideSusie Remembering]

Because we're all different I suppose. I'm one who is on Levo plus T3 and I need a good level of FT4 as well as FT3, I need them fairly well balanced, around 70% through range. If my FT4 is low in range I am very unwell. But I can't tell you why I'm like that and others are fine with a low FT4.

[helvella Administrator Thyroid UK]

There are several possibilities, but it isn't always entirely clear.

For example, the brain largely manages T4 -> T3 conversion itself. Without T4, it is not able to control the T3 levels by adjusting conversion.

And some tissues, such as hair follicles, are known to do their own conversion locally (though maybe also getting some from the bloodstream).

[jimh111]

Both T3 and T4 are transported into cells. Some tissues such as the brain tend to rely on converting T4 to T3 locally whereas others rely on T3 from the blood. T3 binds to receptors in the cell nucleus to activate gene expression. This is the main function of thyroid hormone but there are other 'non-genomic' actions some of which have T4 as the main hormone.

Although I managed OK for several years on high dose L-T3 (hence no T4) I would tend to take some T4 just in case it has actions we are not aware of. My approach would be to try T3 and T4 in combination to match normal blood levels first and then adjust if necessary (I'm not a doctor). Thyroid hormones are incredibly complex, a point missed by most medics.