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Who's doing the negative feedback, T3 or T4?

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What exerts a negative feedback on the hypothalamus? free T3 or T4 levels?
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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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