Palaniappan
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A 54/M presenting with palpitations,nervousness and weight loss.Serum TSH is low.He admits taking levothyroxine for losing weight.What is the most likely cause for decreased TSH??
A.T3 insensitivity
B.Free T3
C.Free T4
D.Protein bound T4.
Please explain your answer. 
Thanx
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| me007
Forum Guru
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protein bound T4 is metabolically inactive.
Should be free T4
C.
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| drplacebo
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B..
Only T3 can regulate feedback control of TSH
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| Ancylostoma
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B I agree with Doctor placebo, I think this was a usmleworld question!
Edited by Ancylostoma on 01/17/07 - 10:51 PM
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| bangledoc
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Both FT4 and FT3 have a negative feedback relationship with TSH.
1)an increase in FT4/FT3 should produce a decrease in TSH
2)a decrease in FT4/FT3 should produce an increase in TSH
Rf.......RR Goljan 2006 ed. 492 page.
Kaplan phy says.........................
The main circulating form is T4,it is T4 that is responsible for most of the negative feedback.
However,within thyrotroph the T4 is converted to T3 before it acts to reduce the sensitivity of the thyrotoph to TRH.
As long as circulating FT4 remains normal,changes in circulating T3 have minimal effects onTSH secretion.However,TSH secretion increases if there is a significant drop in circulating FT4,even in the presence of an increase in circulating T3.
So i wll go with C.
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| Ancylostoma
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Yeah thats what my logic and what I have always been taught, but the reason I picked t3 is I rememebered this wierd usmle world quesiton and i think i picked t4 and got it wrong.
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| me007
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ok,I picked T4 because levothyroxin is T4
but T3 is more biologically active than T4
T3 down-regulates TRH receptors in the anterior pituitary and thereby inhibits TSH secretion
are you guys sure that it is T3?
it might be right answer.
Just want to make sure that i memorize right information.
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| Ancylostoma
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You could easily be right just as well, I had the same logic before and I recall this type of question and I picked t4 and got it wrong. Dont memorise anything until palani comes with the answer!!
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| Palaniappan
Forum Senior
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According to Usmle World,the answer is T3.....It says that T3 has a significant effect more than T4 in regulating TSH..Very good discussion guys
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| Palaniappan
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But...Kaplan Q bank and Kaplan notes says that the decrease is due to T4....It would be better if somebosy post the correct anser for this wierd question....
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| Ancylostoma
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Does anyone have a definative reference ? I will ask my sis if she can look it up in harrisons. I glad you posted this question. I thought it was a little funny. Atleast it contradicted what I was taught in kaplan. Lets see what we can all dig up. You can also write usmle world and ask them for the reference regarding that question. Tell them that something there contradicts what you learned from kaplan. Kaplan says that since t4 is the main hormone in the body, its free amount is the one the regulate that feedback inhibition on TSH. But that it does so throught the conversion by 5' deiodinase in the pituitary. See what they say. And ask if they could refer you to a text.
Edited by Ancylostoma on 01/18/07 - 07:28 AM
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| nandrolone
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The negative feedback is due to increased levothyroxin ie T4. Hence the answer has to be free T4.
Anyway in the nornmal body T4:T3 is 50:1 because T4 has a longer t1/2. and t4 is more likely produs the effect
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| sixfeet
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brs physio says.. T3 down regulates the TRH receptors in ant pitutary...
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| usmle4me
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T3 is more active but has less -ve feedback on TSH,it increases bmr but not affecting TSH and so not affecting the size of gland. so used for rx for obesity.
T4 less active but has more -ve feedback on TSH
I shall pick free T4
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