In our meds ,approx what % of that T4 converts to T3 please, and if you are DIO2+ do we know approx what % of T4 gets converted to T3 ,is it

[Gillybean1]

ny thoughts greatly appreciated,thank you . G

[Jazzw]

I don’t think anyone’s going to be able to answer that—I think it’s different for everyone, which is why GPs get so confused about levothyroxine dosage. It’s also going to depend on the ability of one’s gut to absorb medication and other nutrients—something which again is hugely variable.

[Gillybean1 in reply to Jazzw]

Hi Jazzw, Thank you for taking the time to reply. No i suppose it is very individual, perhaps i should have worded it as 'what % approx of T4 from our thyroid gland is converted to the active T3' , just to get a reference range.

Every best wish , G

[Jazzw in reply to Gillybean1]

Ah! Maybe this helps then?

Quoting from this article: drknews.com/conversion-t4-t...

About 94 percent of the hormone made in the thyroid gland is T4. The remaining 6 percent is triiodothyronine (T3), named for its three molecules of iodine.

These thyroid hormones hitch a ride through the bloodstream on thyroid-binding proteins, during which they are referred to as “bound.” When they are dropped off at the cells for active duty, they are called “free” hormones.

Although the thyroid gland secretes only a little T3, it is the most active form the body can use. T4 must be converted to T3 before the body can use it. Most of this conversion happens in the liver, but also take place in cells of the heart, muscle, gut, and nerves. These cells convert T4 to T3 with an enzyme called tetraidothyronine 5’ deiodinase, which removes one molecule of iodine.

In the end, only about 60 percent of T4 is converted into usable T3. Twenty percent becomes reverse T3 (rT3), an inactive form the body cannot use. Levels of rT3 can become too high in times of major trauma, surgery, or severe chronic illness. Another 20 percent of T4 can be converted to T3 by healthy gut bacteria in the digestive tract.

[helvella Administrator Thyroid UK in reply to Jazzw]

Those figures look very different to the ones I have more often read!

Metabolism of Thyroid Hormone

Robin P Peeters, M.D. Ph.D. and Theo J Visser, Ph.D.

Author Information

Last Update: January 1, 2017.

In healthy humans the thyroid gland produces predominantly the prohormone T4 together with a small amount of the bioactive hormone T3. Most T3 is produced by enzymatic outer ring deiodination (ORD) of T4 in peripheral tissues. Alternative, inner ring deiodination (IRD) of T4 yields the metabolite rT3, the thyroidal secretion of which is negligible. Normally, about one-third of T4 is converted to T3 and about one-third to rT3. The remainder of T4 is metabolized by different pathways, in particular glucuronidation and sulfation.

ncbi.nlm.nih.gov/books/NBK2...

In someone taking levothyroxine, it is usually suggested they only absorb between 50 and 80% of what they take.

Further, it all depends on how you are measuring! Each molecule of T3 is only around 84% the weight of each molecule of T4. Therefore, even if you absorbed and converted ALL of it, you would only end up with 84 micrograms of T3 for every 100 micrograms of T4! (The number of molecules, of course, being the same.)

[Jazzw in reply to helvella]

I did wonder. I started looking for other evidence but didn’t manage to get very far!

Thanks for this.

[Gillybean1 in reply to helvella]

Thank you for this Helvella,

Every best wish G

[Gillybean1 in reply to Jazzw]

Thank you Jazzw, lots to get my head round.

Every best wish G

[tattybogle]

It's complicated !

What % is converted from meds ? .......you can only know that if you ONLY take T4 and have NO functioning thyroid gland at all... otherwise you can measure T3 but have no way of knowing how much is converted from T4 and how much is produced by the thyroid.

What % of T4 from thyroid gland is converted to T3 ?.... it varies a lot between individuals , and it is frequently adjusted within the same person by clever things (deiodinases)

You may find some answers (or explanations of why you can't have an answer) in here :-

thyroidpatients.ca/home/sit... .... a vast collection of information, but here are some suggestions :-

thyroidpatients.ca/2020/05/...

thyroidpatients.ca/2019/06/...

thyroidpatients.ca/2020/05/...

[Gillybean1 in reply to tattybogle]

thank you Tattybogle, lots to digest.

Every best wish, G