doubt..TSH feedback inhibition

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doubt..TSH feedback inhibition

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docdoc9
Forum Senior

Topics: 17
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02/25/08 - 09:26 PM

Feedback inhibition of TSH is by T3 or T4

Kaplan says T4

UW says T3

HNPCC
Forum Newbie

Topics: 0
Posts: 6

02/27/08 - 12:33 PM

It has been now well known that both T4 and T3 provide feedback on TSH. Some references however say that T3 is more powerful. I don't think USMLE asks questions of this nature . I mean where there is controversy around the issue.
Check this old reference if you like.

http://ajpendo.physiology.org/cgi/content/abstrac...

docdoc9
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03/04/08 - 01:58 PM

Thanks HNPCC

new_n_lost
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03/04/08 - 02:28 PM

http://www.prep4usmle.com/forum/thread/56239/

http://www.prep4usmle.com/forum/thread/47694

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docdoc9
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03/04/08 - 03:26 PM

Hey nnl

Thanks for the links

I think the confusion still continues.......

new_n_lost
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03/04/08 - 03:28 PM

yeah it does continue there were other thread which have had the same discussion. But i think so far T4 is still leading the battle. wink

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keepgoing
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03/04/08 - 08:00 PM

its T3,its more active and even if u inject T4(levothyroxin)it would convert into T3 in periphery and its T3--> dec TSH via dec effect of cells to TRH.

antidepressant
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03/09/08 - 01:20 PM

thanks KG,

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macintosh
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03/11/08 - 02:17 PM

T3 is the more active form of thyroid hormone; whereas T4 is the more abundant form in normal conditions. As T3 is the more active form it cause more effects on end organs and tissues than T4. As T4 is more abundant in normal conditions (about 50:1) it is mainly responsible for the negative feedback effect. To explain this further consider 50 molecules of T4 providing the negative feedback to hypothalamus and pituitary as compared to 1 molecule of T3. In the hypothalamus and in pituitary however the T4 is converted to its more active form T3 (just as it does in the peripheral tissues) to cause its effect.

keepgoing
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03/14/08 - 10:10 AM

#10

macintosh wrote:
T3 is the more active form of thyroid hormone; whereas T4 is the more abundant form in normal conditions. As T3 is the more active form it cause more effects on end organs and tissues than T4. As T4 is more abundant in normal conditions (about 50:1) it is mainly responsible for the negative feedback effect. To explain this further consider 50 molecules of T4 providing the negative feedback to hypothalamus and pituitary as compared to 1 molecule of T3. In the hypothalamus and in pituitary however the T4 is converted to its more active form T3 (just as it does in the peripheral tissues) to cause its effect.

agree,but Dr Dunn clearly explains,besides,T4 the most abundant form of TH ,which have negative feedback BUT to have actual negative feedback, T4 enters ANT.pituaitary:-thyrotrops and convert T4 into T3--then negative feedback take place so its not false if we say T3 does have negative feedback.(i liked his explanation).

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