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Old 01-02-2012
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Thyroid Thyroid Hormones; Total versus Free Levels!

hi
in one of uworld questions , it says that increase estrogen lead to increase thyroid binding globulin which lead to increase total t4 and total t3 while free t3 remain the same,
but why since TBG is increase free T3 should decrease as more amount will bind to TBG and the total remain the same
I just cant get it
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Old 01-02-2012
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free T3 stays the same and the patient remain euthyroid because :

when TBG increase---> bind more free T3 ---> more and more T3 becoming bound forms ----> decrease in free T3 (initially) ----> removal of the -ve feedback inhibition on pitutary gland --> increase TSH --->increase free T3 back to normal
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when TBG increases--more T3,T4 is bound leading to increase in total T3,T4 and there is decrease in free T3,T4and--so more free T3 is released from follicles to maintain the free T3,T4 levels or else pt will be hypothyroid and every pregnant woman will be hypothyroid.
also the loss of negative feedback on pituitary helps to produce more T4,T3 in follicles and thus maintaining the levels constantly.
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Old 01-03-2012
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Quote:
Originally Posted by doctordexter View Post
when TBG increases--more T3,T4 is bound leading to increase in total T3,T4 and there is decrease in free T3,T4and--so more free T3 is released from follicles to maintain the free T3,T4 levels or else pt will be hypothyroid and every pregnant woman will be hypothyroid.
correction...i am not sure abt the negative feedback.cause the TSH is normal as the free T4T3 levels are maintained the same.
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Old 01-03-2012
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Quote:
Originally Posted by tiger73jo View Post
free T3 stays the same and the patient remain euthyroid because :

when TBG increase---> bind more free T3 ---> more and more T3 becoming bound forms ----> decrease in free T3 (initially) ----> removal of the -ve feedback inhibition on pitutary gland --> increase TSH --->increase free T3 back to normal

thank you that is great short and clear answer
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