What is Resistance caused by RT3
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When we talk about "resistance to thyroid hormone" we are talking about people that don't respond properly to Thyroid Hormone supplementation and appear to have hypothyroid symptoms despite being on thyroid replacement and having normal blood test results.

One of the most common causes of this lack of response to the thyroid hormone is something called "Reverse T3" (RT3).

At a very basic level thyroid hormone is made of iodine among other things and the numbers that are talked about (T4 and T3 etc) represent the number of iodine atoms in the molecule.

T4 is the main hormone that is produced by the thyroid gland and supplemented "Synthroid, Levoxyl, Thyroxin etc", This hormone is totally inactive biologically and it's only after an iodine atom is removed making it into T3 that it has an effect on the body.

T4 is a storage hormone and lasts for weeks  in the body, T3 is the active one that deiodinates to T2 and then further to T1 which also have metabolic activities of their own. T3 has a life of a few days in the body.

T3 is a catalyst that the body needs for it's chemical reactions to progress at the right speed, without enough of it people are cold, tired, and lethargic.

Reverse T3 (RT3) is what is made when the "wrong" iodine atom is removed from T4, it's a "mirror image" molecule to T3 and is not bio-active. This in itself is not a problem, the problem is that in excess it "fits into the T3 receptors" and gets stuck there blocking the action of T3 on the body. This means  that your body doesn't respond properly to T3 leading to hypothyroid symptoms despite a normal TSH and normal T3 and T4 levels in the body. This is what we refer to as "Tissue Resistance to Thyroid Hormone". Doctors will tell you this is very rare, this   is because they don't look for it!!

Fortunately there are ways of treating it and people can recover and feel well again.