is TSH inhibited by T3 or T4?

[dingleberry007]

hello everyone...

I am a little confused about this topic and was hoping someone could please help me out.

according to kaplan physio, TSH feedback inhibition is mainly due to T4 levels.

however, in a question in UW, this patient had low TSH because he was taking levothyroxine pills and the answer to what was causing the low TSH was T3 levels.

can someone please explain this discrepancy to me?

thank you very much

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[hj0517]

Please check this as I'm unsure myself: its T3 because T3 is predominantly internalized by the thyrtroph cells in the hypothalamus and has the effect of decreasing TRH and so inevitably decreases TSH.

Once again, my credentials are unproven at best and I will probably get slaughtered for saying the above on here so try and look it up - i'm sure you'll find something similar to what I've said.

 

[RussianJoo]

I always thought that t3 was the active form and that t4 is converted to t3 in the periphery. that's why t3's feedback inhibits both TRH's release and TSH's release directly. anotherwords, both TRH and TSH release is effected individually, and since TRH is low, TSH release is effected by that as well.

 

[shivasHeroLike]

yup i agree with the previous two posters.