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Both act on the hypothalamus and the anterior pituitary inhibiting CRH and T4 release. Remember that only FREE forms of T3 and T4 will act negatively, not the ones bound to serum TBG. The latter is useful in pregnant patients as well those who take OCPs who have increased total T4 and T3 but the free portions are unchanged (=euthyroid state)

Source: Williams Textbook of Endocrinology, 11th ed
 

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only t3 will have negative feedback, t4 goes there and its converted ONSITE by 5'mono deiodinase to form t3 which causes negative feedback of tsh.

source : kaplan physio book!
AFIK - there are two levels of inhibition at the TRH level and at the TSH level.

at TSH level : inhibited by BOTH t3 and t4, however, since t4 is much greater in amount than t3, it is the lead in inhibiting.

at TRH level : as mentioned t4 being converted into t3 for "sensing".

Goljan mentions that T4/T3 RATIO might be the best feedback mechanism for TSH.

In cases of iodine deficiency in the environment, lower amounts of T4 might be secreted while the levels of T3 might be normal... even in those situations TSH is up..

so id say T4 - specifically T4/T3 at Pituitary level and T3 (via T4) at hypothalamus. :eek:

hope im right tho.. sort of confusing.. :rolleyes:
 
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