Been trolling through a couple of websites re thyroid issues etc. Came across this paragraph. I thought I had read conversion occurred mostly if not all in the liver.
“Without a thyroid, the body cannot produce the critical thyroid hormones T4 and T3. Furthermore, conversion of T4 into T3 primarily takes place in the thyroid (conversion can occur in the peripheral tissues but it is not nearly as efficient). Without a thyroid gland, one will almost assuredly be T3 deficient. For this reason, it is important for hypothyroid patients without a thyroid to be prescribed treatments that include T3 in addition to T4.”
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NickP1
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I couldn't answer about the proportion of conversion, but diogenes has posted/responded about this question several times recently. I suggested you have a look at his profile and read what he has posted/replied.
This means that in health the thyroid doesn't just supply T4 for the body to make T3 but also controls the overall T3 production by finetuning the system overall against unwanted surges.
I think (and am not medically qualified) it is about having no thyroid gland at all.
This is from the following link:-
Liver Issues. About 20% of the conversion of T4 to T3 takes place in the liver, and so if you don’t have a properly functioning liver then this can also cause problems with the conversion. And just because the liver enzymes look fine on a blood test doesn’t mean that you don’t have any liver problems. While these tests can be helpful to rule out any serious liver conditions, if someone’s liver has a reduced ability to detoxify due to other factors, such as all of the toxins we’re exposed to through the food we eat and the environment, then this can reduce the liver’s ability to convert T4 to T3. So if someone has a problem converting T4 to T3, then a liver detoxification program may be beneficial.
.......and
So while a very small amount of T3 is produced by the thyroid gland, the rest of the T3 is produced in different parts of the body, with the liver responsible for a good amount of this conversion. However, in order to convert T4 to T3, the enzyme 5′ – deiodinase is required. And certain minerals are required to activate this enzyme.
Both the liver and thyroid express type-1 deiodinase (D1) which converts T4 to T3 and rT3. Most circulaing T3 is produced by D2 which converts T4 to T3 and is also responsible for local control of tissue T3 levels.
I understand that some T3 conversion happens in the gut, which may have longstanding digestive issues (IBS/SIBO/food intolerance and inflammation preventing this.
Also, some people (50/50% got it or not?) do not have the D102? gene and therefore, are unable to make the hormone T3. As the NHS is charged a ripoff price for synthetic T3, Drs deny that you need it prescribing, are reluctant to test for low T3 (saving the cost of the test) and don't seem to know about the D102 missing gene, thereby saving that cost too.
The teaching for thyroid conditions is decades out of date, covers only the basics and ends with the assumption that if you are prescribed synthetic Levo, you are "fixed", therefore all continuing hypo symptoms can be ignored and your inevitable weight gain, exhaustion, aches, digestive misery, depressive brain fog and low calorie extremely healthy diet, are the result of "your overeating junk food and laziness". Yes, I've been told that, also repeatedly "I can't help you". We just don't have a 'fashionable' illness !
I think the gut plays a very minor role in deiodinase (conversion of T4 to T3). The DIO2 polymorphism is also quite minor, it causes small differences in cognitive function in patients on levothyroxine only therapy but the difference in conversion is so small it cannot be detected in blood tests. Patients with the DIO2 polymorphism should receive a little liothyronine to replace the 6 mcg their thyroid usually secretes.
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What exactly is Thyroid Peroxidase?
Sorry no link, it's from an ad site, but still a good read: Improving Thyroid Hormone Conversion
Where does T4 to T3 conversion take place?
