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Old 01-31-2012
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Thyroid What controls negative feedback of thyroid hormones?

FA (pg. 295) says T3
Kaplan (pg. 329) says T4

Which is it?
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Old 01-31-2012
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Yeah, that's confusing ... But here's the real deal -->

T4 is pretty much a circulating PROHORMONE of T3.

The main negative feedback of thyroid hormones is exerted mainly at the elevel of the anterior pituitary gland - MOSTLY by T4 (since its the main circulating hormone).

I said MOSTLY because T4 once taken up by the anterior pituitary is converted to T3 before it acts to reduce the sensitivity of the thyrotroph cells to TRH. But its still T4 that is considered to have the main negative feedback...well for the exam purposes anyway!

There is some inhibition at the hypothalamus as well but not significant.



I hope this helps.
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Old 01-31-2012
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Originally Posted by Hope2Pass View Post
Yeah, that's confusing ... But here's the real deal -->

T4 is pretty much a circulating PROHORMONE of T3.

The main negative feedback of thyroid hormones is exerted mainly at the elevel of the anterior pituitary gland - MOSTLY by T4 (since its the main circulating hormone).

I said MOSTLY because T4 once taken up by the anterior pituitary is converted to T3 before it acts to reduce the sensitivity of the thyrotroph cells to TRH. But its still T4 that is considered to have the main negative feedback...well for the exam purposes anyway!

There is some inhibition at the hypothalamus as well but not significant.



I hope this helps.
hey, if the steps like t4, so do i.

thanks!
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Old 02-01-2012
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it is t3 the main control , but T4 is converted to T3 in hypothalamus and the T3 is the factor that really affect the hypothalamus and pituitary , according to UWORLD
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Originally Posted by rose View Post
it is t3 the main control , but T4 is converted to T3 in hypothalamus and the T3 is the factor that really affect the hypothalamus and pituitary , according to UWORLD
kaplan says t4 though :/

maybe somebody has a 3rd source? i dont want to get such a simple detail wrong.
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Old 02-01-2012
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T3 is the dominant active form
So only active form gives most DOMINANT feedback!

FA is correct here!
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Hey I just took a Kaplan course and they taught us that its T4

What should I go with ?
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