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  1. #1
    hunteradam07 is offline Permanently Banned 511 points
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    Endocrine question!

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    A 42 years old female, who works in hospital came in with excessive sweating, weakness, heat intolerance, and recent weight lost. During examination, she admitted to take Levothyroxine to lose weight recently, and her lab result shows decease in TSH. What is the reason for her decrease in TSH?

    A. Free level T3.
    B. Free level T4.
    C. Inhibin.
    D. Prolactin.
    E. ACTH.
    F. Cortisol.
    G. ADH.

    I know the answer but i m interested in why? Because there seem to be a discrepency btw Kaplan's notes and UW answer on this. Thanks.

  2. #2
    student-2's Avatar
    student-2 is offline Senior Member 510 points
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    Probably free T4- T3 has a short half life and T4 is the main form of thyroid hormone in the body although I would think that T3 can also have an effect
    Also Levo is T4

  3. #3
    HHN786 is offline Member
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    I thought T3 is involved with the negative feedback and causes low TSH, is that true?

  4. #4
    bpoichan is offline Member
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    answer is T4

    T4 is the one involved in negative feedback...for the simple reason that the T4:T3 ratio is 50:1
    you may think it is 20:1 ..which is true when T4 and T3 are secreted from the follicle...but since T4 has 6 days half life and it binds VERY strongly to TBG...its ratio compared to T3 changes to 50:1

    now.. T4 being soo numerous is the one having a negative feedback on TSH secretion... although i should say that once in the thyrotrophs in the anterior pituitary...T4 is converted to T3 before playing its negative feedback role..
    nevertheless T4 is the one playing a negative feedback..

    There are TOO few T3 for them to play a negative feedback..

    if you need to understand more.. just consider the Low T3 syndrome...
    Decreased T3, NORMAL TSH and T4.....
    if T3 had a negative feedback TSH would not be normal but it is in the Low T3 syndrome...meaning that T3 does not have a negative feedback role but T4 does..

    hope it helped..

  5. #5
    student-2's Avatar
    student-2 is offline Senior Member 510 points
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    oooops

    Hate to admit it but I made a mistake- I t didn't feel 100% sure with my previous response so I looked it up- T3 is responsible for negative feedback on Ant. Pituitary secretion of TSH. Both bpoichan and I were wrong.
    Hate to admit it but I made a mistake. Happens!

  6. #6
    hunteradam07 is offline Permanently Banned 511 points
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    Well the answer according to USMLEWorld is Free T3, and their explaination is Free T3 is the most important negative feed back mech TSH-That is all it said-I pick free T4, because i read in Kaplan Physio notes about the ratio and how T4 bind stronger to TBG too. So that is why i m confused now. Although, I didnt know that T4 will convert to T3 in ant Pit, which down regulate TSH. Maybe that is what they mean. Anyone else having an opinion?

  7. #7
    bpoichan is offline Member
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    ok...

    I still believe it is T4

    if it was T3...then could anyone explain me why in the ' Low T3 syndrome'...that occurs in chronic diseases...is the TSH normal?
    if T3 really had a negative feedback....then in Low T3 syndrome.. TSH SHOULD BE ELEVATED but it is not!

    we can also notice that T4 is normal in the Low T3 syndrome...therefore a normal T4 and normal TSH....with a low T3 ... can only mean that T4 has a negative feedback on TSH.

    Merck Manual 7th edition page 83 says:
    ' pituitary TSH secretion is controlled by negative feedback mechannism modulated by the circulating level of free T4 and free T3 and by conversion of T4 to T3 in the pituitary thyrotropic cells..'

    therefore BOTH T3 and T4 have a negative effect...but the question forces us to one single answer and the most logical one seems to be T4

    that is what i believe! correct me if i am wrong..

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    hunteradam07 is offline Permanently Banned 511 points
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    I dont know either hopefully someone can really have a clear answer. Calling all Physiologist! Help!

  9. #9
    bpoichan is offline Member
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    Believe or not, I just emailed Dr Goljan HIMSELF!!
    hope he ll answer..if not then we ll have to look somewhere else for a confirmatory answer...
    but i am still sticking to my T4 answer!

  10. #10
    HHN786 is offline Member
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    Tttttttttttttttt33333333333333333

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