TSH feedback T3 or T4?

[axeon123]

---

Members do not see this ad.

kaplan = T4
FA 2009 = T3
uworld = administration of levothyroxine suppresses TSH b/c it's like high T4

Any input? I'm thinking T4 b/c it is the molecule with longer half-life and you use it up if you run low on T3? And the half-life of T3 is super short, 1day.

 

[Domenech]

Well I have kind of learned not to over-trust FA on controversial issues like these. I'd put my money on T4 as this is the molecule present in higher concentrations and for a longer period in physiological conditions.

 

[StIGMA]

Harrison's figure 335-2 states that both T4 and T3 feedback to inhibit TSH and TRH. The molecules are extremely similar in structure; don't you think they would bind the same feedback receptors?

 

[turkeyjerky]

Harrison's figure 335-2 states that both T4 and T3 feedback to inhibit TSH and TRH. The molecules are extremely similar in structure; don't you think they would bind the same feedback receptors?

yeah, T4 isn't completely devoid of biological activity. But the main feedback mechanism is T4 being converted into T3 w/in the anterior pituitary.

 

[PokerDoc]

my understanding was that both T3 and T4 inhibit TSH but that T3 also inhibits TRH and thus has a stronger overall inhibitory ability.

I really wish I had my goljan with me, cuz im pretty sure thats where I read it.. it was either HY Biochem or BRS physio.

In 3 hrs I'll have my books again and I'll double check.

 

[DiverDoc]

my understanding was that both T3 and T4 inhibit TSH but that T3 also inhibits TRH and thus has a stronger overall inhibitory ability.

I really wish I had my goljan with me, cuz im pretty sure thats where I read it.. it was either HY Biochem or BRS physio.

In 3 hrs I'll have my books again and I'll double check.

I got a question about this regarding what inhibits TRH, and the correct answer was T3.

Also, It said this was the 'best' way to eval hypo vs. hyper thyroid. But I thought a standard lab test of T3, T4 and TSH would tell you that.

 

[TexasTriathlete]

I got a question about this regarding what inhibits TRH, and the correct answer was T3.

Also, It said this was the 'best' way to eval hypo vs. hyper thyroid. But I thought a standard lab test of T3, T4 and TSH would tell you that.

I feel like I second-guess myself all the time on this stuff.

I got the UW question too saying it was T3 that is the main feedback on TSH.

And I think TSH is the best way to evaluate hypo/hyper thyroid.

 

[username456789]

TSH is basically always going to be the best single screening test of thyroid function. And when you're following a patient who is, say, hypothyroid and taking synthroid, it's pretty common to just check a TSH.

And I believe we learned in class that T3 is the most potent feedback regulator, but I'm not sure we made a distinction between hypothalamus and pituitary feedback.