Re: Does TSH reflect conversion of T4 to T3? - BUMP

[Guest guest]

Do we really not know the answer to this basic question?

Miriam

> Is this the one you were referring to? It was me who picked out the

> following sentence (from a letter by an endo)and asked if it was true.

Unfortunately it went unanswered and is now on the third page somewhere, so if

anyone does know the answer, I'm still interested - and so is Miriam []

>

> " She is well informed over normal thyroid physiology and understands

> that Tri-iodothyronine is the active thyroid hormone and that in

> patients receiving Thyroxine exclusively, normal T3 levels are

> maintained by peripheral conversion of Thyroxine. I have explained to her that

the pituitary thyroid hormone receptor is a receptor

> exclusively for T3 and that the maintenance of a normal TSH on Thyroxine

replacement is therefore dependant on the conversion of T4 to T3. "

>

> Gill

[Guest guest]

Hi Miriam and Gill,

It was myself who was told this by an Endo a couple of weeks ago. While I am still learning about my thyroid and the (what seems like) vast amount of external factors which affect can affect so many different things, which in all honesty just totally confuse me. I have to keep reading and re-reading things in the hope that I manage to read it during an all too brief period of lucidity where my brain can actually absorb the information. I don't count myself knowledgable enough to be able to explain anything to do with the thyroid but I have come acroos the following link which even I managed to understand, a bit like a eureka moment it was to be honest.http://tiredthyroid.com/tsh.html

One wee section I found especially interesting is the following:

"Anyone with Graves' disease who has had radioactive iodine treatment (RAI), a thyroidectomy, or takes anti-thyroid drugs (ATD), and is now on thyroid medication, should not be dosed by TSH. Graves' is caused by TSH Receptor antibodies, and TSH can stay suppressed (near zero) for months or longer, even after treatment brings thyroid hormone levels down into the normal range or even below normal. [4] If they are unable to get adequate thyroid hormone replacement because of their suppressed TSH, these patients will suffer from hypothyroid symptoms. They would fare much better by adjusting their dose to Free T3 and Free and/or Total T4 levels, instead of the TSH."

This is me - so why do the GP and Endo just look at TSH? I have never once had any form of testing whatsoever done regarding T3 and they refuse point blank to do these tests. It just proves that they really don't have any clue whatsoever sadly.

Lynn

>> Do we really not know the answer to this basic question?> Miriam

> > "She is well informed over normal thyroid physiology and understands> > that Tri-iodothyronine is the active thyroid hormone and that in> > patients receiving Thyroxine exclusively, normal T3 levels are> > maintained by peripheral conversion of Thyroxine. I have explained to her that the pituitary thyroid hormone receptor is a receptor> > exclusively for T3 and that the maintenance of a normal TSH on Thyroxine replacement is therefore dependant on the conversion of T4 to T3."

[Guest guest]

Thanks, that's a good article.

This part seems particularly relevant: " One study determined that the relative

potency of T3 to T4, in terms of its ability to inhibit TSH, was 100:12. [8] "

That means a low TSH might reflect conversion of T4 to T3. But given that there

are so many other factors which can lower TSH, it is not a reliable guide.

Miriam

>

> It was myself who was told this by an Endo a couple of weeks ago. While I am

still learning about my thyroid and the (what seems like) vast amount of

external factors which affect can affect so many different things, which in all

honesty just totally confuse me. I have to keep reading and re-reading things in

the hope that I manage to read it during an all too brief period of lucidity

where my brain can actually absorb the information. I don't count myself

knowledgable enough to be able to explain anything to do with the thyroid but I

have come acroos the following link which even I managed to understand, a bit

like a eureka moment it was to be honest.

> http://tiredthyroid.com/tsh.html <http://tiredthyroid.com/tsh.html>

[Guest guest]

hello lynn, gill and miriam

i found the information about deiodinase enzymes to be absolutely crucial in

understanding how the tsh works...and more particularly, understanding the knock

on effect of too much stress (from any source, even too much fun!) i.e.

initially to produce too much cortisol but after a prolonged period to not be

able to produce enough cortisol...both situations impede thge use of t3 ...the

way this occurs is by the body's changing production of these important enzymes

in response to stress.

i first came across this link about 15 months ago and it has probably taken me

that long to really get to grips with what it is saying!

http://nahypothyroidism.org/deiodinases/

" To accurately assess thyroid function, it must be understood that deiodinase

enzymes are essential control points of cellular thyroid activity that determine

intracellular activation and deactivation of thyroid hormones.

This local control of cellular thyroid levels is mediated through three

different deiodinase enzymes present in different tissues in the body; type I

deiodinase (D1) and type II deiodinase (D2) increase cellular thyroid activity

by converting inactive thyroxine (T4) to the active triiodothyronine (T3) while

type III deiodinase (D3) reduces cellular thyroid activity by converting T4 to

the anti-thyroid reverse T3 (reverse T3) (1-9) (see deiodinase figure).

The activity of each type of deiodinase enzyme changes in response to differing

physiologic conditions, and this local control of intracellular T4 and T3 levels

results in different tissue levels of T4 and T3 under different conditions.

Because it is the activity of these deiodinases and transport of T4 and T3 into

the cell that determines tissue and cellular thyroid levels and not serum

thyroid levels, serum thyroid hormone levels may not necessarily predict tissue

thyroid levels under a variety of physiologic conditions.

Depression

Many depressed and bipolar patients have undiagnosed thyroid dysfunction as the

underlying cause or major contributor to their depression that is not detected

by standard thyroid tests (23-38). The dysfunction present with these conditions

includes down regulation of D1 (reduced T4 to T3 conversion) and reduced uptake

of T4 into the cell, resulting in increased serum T4 levels with low

intracellular T3 levels (24-26,30,31,35,39-45) and upregulated D3, resulting in

elevated reverse T3 (23,24,30,31), which blocks thyroid effect (147,184-194) and

is an indicator of reduced transport of T4 into the cell (183,193).

Additionally, studies show that depressed patients have reduced T4 transport

across the blood brain barrier due to a defective transport protein,

transthyretin, resulting in significantly reduced thyroid levels in the brains

of depressed patients despite “normal†serum levels and standard thyroid

tests (23,39,40) as well as a reduced TSH response to TRH (28-31,43-50).

It is not surprising that T4 and T4/T3 combinations may have some benefit in

depression; but due to the suppressed T4 to T3 conversion from suppressed

D1(24-26,30) and reduced uptake of T4 into the cell and brain (25,31,39,40),

timed-released T3 is significantly more beneficial than T4 or T4/T3 combination

supplementation (25,41,202,225-227). "

here's another link about these enzymes.

trish

>

> Hi Miriam and Gill,

>

> It was myself who was told this by an Endo a couple of weeks ago. While

> I am still learning about my thyroid and the (what seems like) vast

> amount of external factors which affect can affect so many different

> things, which in all honesty just totally confuse me. I have to keep

> reading and re-reading things in the hope that I manage to read it

> during an all too brief period of lucidity where my brain can actually

> absorb the information. I don't count myself knowledgable enough to be

> able to explain anything to do with the thyroid but I have come acroos

> the following link which even I managed to understand, a bit like a

> eureka moment it was to be honest.

> http://tiredthyroid.com/tsh.html <http://tiredthyroid.com/tsh.html>

[Guest guest]

this is the other link ....sorry!

http://www.jci.org/articles/view/29812

> here's another link about these enzymes.

[Guest guest]

Thanks. I'll have a look at those.

Miriam

> > here's another link about these enzymes.

>