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Old 07-21-2012
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Thyroid TSH negative feedback?

Which among the following is responsible for negative feedback of TSH?

A) Free T3
B) Free T4
C) T4/T3 ratio

I dont know the answer. found conflicting opinions all over the internet.
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Old 07-21-2012
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Previously discussed here
What controls negative feedback of thyroid hormones?

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At receptor level it's T4
At nuclear level (gene transcription) it's T3
This is because once the T4 enters the Pituitary cell it gets converted to T3

So if they specify the level you can answer, if they don't specify the level (receptor versus nucleus) then pick T4 because it's the main circulating hormone.

By the way there's a lot of debate about this issue and scientist still in controversy about it. This means USMLE questions makers would not ask about it.
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Quote:
Originally Posted by Hokolesqua View Post
At receptor level it's T4
At nuclear level (gene transcription) it's T3
This is because once the T4 enters the Pituitary cell it gets converted to T3

So if they specify the level you can answer, if they don't specify the level (receptor versus nucleus) then pick T4 because it's the main circulating hormone.

By the way there's a lot of debate about this issue and scientist still in controversy about it. This means USMLE questions makers would not ask about it.
bro i thought Free -T3 causes negative feed back.

thats what was mentioned in UWorld.

and free T3 is active form

correct me if i am wrong
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Old 07-21-2012
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this is the correct explanation but if the question is not mentioning clearly, i would choose T3
Quote:
Originally Posted by Hokolesqua View Post
At receptor level it's T4
At nuclear level (gene transcription) it's T3
This is because once the T4 enters the Pituitary cell it gets converted to T3

So if they specify the level you can answer, if they don't specify the level (receptor versus nucleus) then pick T4 because it's the main circulating hormone.

By the way there's a lot of debate about this issue and scientist still in controversy about it. This means USMLE questions makers would not ask about it.
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Old 07-21-2012
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Free T4 has negative feedbck on TSH
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Old 07-25-2012
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Source KLN : free t4
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262/261/PASS 1st attempt/238- USCE 1.5 mo (Uni hospital). YOG -2013; home country resident for 6 mo
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