Explanation of what *high TSH is telling us when our fT4 level is ''normal' on Levothyroxine ~ the Shoe Size analogy. (* over 2.5/3 ish )

[tattybogle]

I thought i would make a new post just to store this explanation, since several people have found it helpful, and we use the 'shoe size' analogy on the forum to explain this quite often.

i was asked this question recently ~ "It’s generally accepted that TSH regulates/controls thyroid production of T4/T3 but my T4 level is normal at 14 [12-22] , so why is my TSH still raised ?"

TSH (Thyroid Stimulating Hormone ) is a 'message' from the Hypothalamus/Pituitary to the Thyroid . (HPT axis)

Your High TSH is saying ....."the amount of T4 we have is not enough yet ".. so it asks the thyroid to make more . However your thyroid is a bit knackered , it can't manage to respond to that request well enough anymore (hence you being put on Levothyroxine)

Once someone with a knackered thyroid (that can't make enough T4) is taking T4 from Levo , then high TSH is not so much a message to their thyroid ...but rather a message to their GP asking them to increase the T4 by increasing the dose.

Your TSH can try asking your thyroid for "more T4 please' , and shout as loud as it likes ( high TSH is 'shouting') .... but your thyroid cant manage to make enough T4.

When your dose is 'enough' for you .. then your TSH will go down ( stop shouting)

just because an fT4 of 14 [12-22] is 'in the normal range' that doesn't mean it's 'enough' for you

.. 14 is only enough for the person who usually has 14.

it is not enough for the person who usually has fT4 of 20 [12-22]

..... it's like shoe sizes .

size 5 is a 'normal' shoe size for women .... and so is size 7

.. the normal range for women's shoe sizes might go from [4 to 8]

But 'normal' size 5 shoes are no use to you if you have 'normal' size 7 feet. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

TSH 'asks' the thyroid to make more/ or less T4/T3 ( FEEDFORWARD) .

and TSH is also a reflection to how much T4/T3 there is in the blood and how satisfied the hypothalamus / pituitary are with that level ( FEEDBACK)

BUT... that is how the system works when the thyroid is making all the T4 ( AND a variable amount of ready made T3)

Unfortunately the Hypothalamus /Pituitary are a bit thick ~ they can't tell the difference between T4 from the thyroid..... and T4 from a tablet.

once we are taking 'enough' of the T4 we need from a tablet.. the TSH goes down again so the thyroid is effectively turned 'off' (or at least 'turned right down' ) and so the FeedForward part of this conversation is less use .... the knackered Thyroid has largely 'left the conversation' once we are taking a 'full' dose of T4

GP's still use the FeedBack part of the conversation as a monitor of how 'satisfied' the body is with the dose of T4 taken (and how much T3 is being made from it) .

However it is not 'foolproof' to use just the TSH FeedBack as a monitor for how satisfied the whole body is once we are taking T4 only .... because it ignores the fact that TSH is ONLY a reflection of how satisfied the Hypothalamus / Pituitary are ... some other parts of the body may not be so happy with this new balance of "more T4: less T3"....... and the TSH can't reflect how the rest of the body feels , it is ONLY in conversation with the Hypothalamus , Pituitary and Thyroid (HPT axis)

The Hypothalamus / Pituitary may be saying "that's enough T4 thanks" , or even "that's a bit too much T4"......... but the muscles, or heart, or liver, or brain etc may be saying "but I haven't got quite enough T3 any more "

[RedApple Administrator]

Excellent tattybogle! When's your thyroid e-book coming out?

[tattybogle in reply to RedApple]

This is it

[SarahJane1471]

👏that is a great explanation Tatts

[Tythrop]

After reading this excellent note I thought of some of my school teachers ..stay with me if you can be bothered. I thought of the Pituatory/Hypothalamus as a teacher and the body as the pupil. So teacher gives the lesson on,a particular subtopic in the syllabus ( sending out the standard amount of tsh,) and goes away assuming that they have done their job by giving out the lesson, ( the feedforward) .This will be fine for the majority of pupils ( thyroids and bodies) BUT NOT ALL because it is normal for there to be some pupils in a cohort with differing or compromised learning methods and speeds / thyroids/ bodies. .Only by getting feedback by doing the fortnightly tests can the teacher find if the tsh /lesson given out is suitable for the whole cohort .Hence the need for feedback by Pituatory/ reHypothalamus reviewing the quantity of t4/3 ( or midterm tests in the case if a teacher) This feedback/ review process judges the need more learning sessions /increase quantity of t4/3 in system.

The fact that the human body has evolved with a feedback system actually proves that there are no rigid fixed amounts of tsh for the human body and why sometimes more t4 etc is needed .So what do doctors insist on the fixed tsh range dogma ?? Now medical science has evolved to make t4 t3 medication ,why not use it appropriately ?

[AmandaK]

Thank you, this is very useful.

[SleepingDisco]

This is amazing! Great explanation that I've now got my head round! Thank you 😊 x

[TaraJR]

tattybogle your shoe analogy is so good!

I help to run ITT Improve Thyroid Treatment group on facebook and we're thinking of things to share on World Thyroid Day. Would you be ok if I share this please?

[tattybogle in reply to TaraJR]

yes , you're welcome to use it however you want Tara .

mind you it's not my analogy.... i pinched it off pennyannie i think , who probably pinched it off Lalatoot who .... well anyway it's been doing the rounds.

[TaraJR in reply to tattybogle]

Love it! The best things do certainly do the rounds!

[pennyannie in reply to tattybogle]

I'm not claiming the fame for my size 9 shoes -

I think this one is " soles " and hands down to Lala - but we never had such graphics before - and they make the statement so strongly - all on their own - !!

I much prefer the roundabouts, swings and see saws in the playground :

So there's a heads up - have a nice day: