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  #1  
Old 10-16-2012
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Thyroid Question on T3 and T4

I had a question which stated:

In a healthy patient, an injection of which substance would DECREASE the patient's T4 levels?

A. Thyroglobulin
B. T3
C. T4
D. Reverse T4

I'll give my reasoning after I see some responses Thanks!
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Old 10-16-2012
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I would go with T3
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Quote:
Originally Posted by Lara View Post
I would go with T3
Reasoning? I picked that as well, but doesn't T4 play the main regulatory role of thyroid hormone... e.g., T4 plays a regulatory role mainly when it's converted intracellularly to T3...
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  #4  
Old 10-17-2012
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Quote:
Originally Posted by candor View Post
I had a question which stated:

In a healthy patient, an injection of which substance would DECREASE the patient's T4 levels?

A. Thyroglobulin
B. T3
C. T4
D. Reverse T4

I'll give my reasoning after I see some responses Thanks!
As i remember, when body is in stress. To conserve energy it make reverse T3 instead of T3 from T4.

In this case, May be T3 increase gives negative feedback and lead to decrease in both T3 and T4.
but not sure
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Old 10-17-2012
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Quote:
Originally Posted by candor View Post
Reasoning? I picked that as well, but doesn't T4 play the main regulatory role of thyroid hormone... e.g., T4 plays a regulatory role mainly when it's converted intracellularly to T3...
So you mean T4 causes decrease in T4????
How? by negative feed back?
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Old 10-17-2012
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Quote:
Originally Posted by candor View Post
Reasoning? I picked that as well, but doesn't T4 play the main regulatory role of thyroid hormone... e.g., T4 plays a regulatory role mainly when it's converted intracellularly to T3...
If we give T3 to the patient then it can decrease TSH via feedback mechanism leading to T4 decrease IMO. T3 is the main regulator of TSH at intracellular level.
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Quote:
Originally Posted by koolkiller88 View Post
As i remember, when body is in stress. To conserve energy it make reverse T3 instead of T3 from T4.

In this case, May be T3 increase gives negative feedback and lead to decrease in both T3 and T4.
but not sure
Wouldnt T3 increase if the body needs energy during stress?Increase T3 increases BMR to cope for stress.
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Old 10-17-2012
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Quote:
Originally Posted by candor View Post
Reasoning? I picked that as well, but doesn't T4 play the main regulatory role of thyroid hormone... e.g., T4 plays a regulatory role mainly when it's converted intracellularly to T3...
Man even if T4 play a regulatory role as u claim, giving T4 would increase T4 anyway.

So the answer definitely is T3 which inhibits further TSH and TRH secretion leading to decrease in T4 secretion from thyroid
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  #9  
Old 01-11-2013
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Default thyroglobulin

can't it be thyroglobulin bcz its not exposed to ciculating immune cells and when its given in circulation it leads to autoimmune response to thyroid leading to gland destruction and low t4???
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  #10  
Old 01-12-2013
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Quote:
Originally Posted by candor View Post
I had a question which stated:

In a healthy patient, an injection of which substance would DECREASE the patient's T4 levels?

A. Thyroglobulin
B. T3
C. T4
D. Reverse T4

I'll give my reasoning after I see some responses Thanks!
i think T4 because main circulating form is T4 and it is responsible for the negative feedback
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i think T4 because main circulating form is T4 and it is responsible for the negative feedback
Physio concept: as long as circulating free T4 remains normal,changes in T3 have minimal efect on TSH secretion, however TSH secretion increases if liltle drop in T4,even in presence of an increase in circulating T3.
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Old 01-12-2013
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Default T4

I'll go with T4 coz T4 is responsible for most of the -ve feedback.
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  #13  
Old 01-12-2013
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Quote:
Originally Posted by candor View Post
I had a question which stated:

In a healthy patient, an injection of which substance would DECREASE the patient's T4 levels?

A. Thyroglobulin
B. T3
C. T4
D. Reverse T4

I'll give my reasoning after I see some responses Thanks!

I will go for B. It's about Negative feedback mechanism.
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  #14  
Old 01-12-2013
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Quote:
Originally Posted by Doctor Ali View Post
I will go for B. It's about Negative feedback mechanism.
T3 down regulates TRH and therefore inhibits TSH. TSH secretes T4 and T3. T4 is highest in populaton.; whereas, T3 is little. However, T4 converts into T3. T3 itself is more potent; meaning active compare to T4. Therefore, TRH and TSH are regulated by T3.

When T3 is low, TSH will secrete T4 and T3. T4 will convert to T3. T3 will raise up to norma levelsl.

When T3 is high, it will inhibit TSH. TSH will not secrete T4 and T3. T4 will not convert to T3. T3 levels will drop down to normal levels.
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Old 01-15-2013
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Quote:
Originally Posted by Doctor Ali View Post
T3 down regulates TRH and therefore inhibits TSH. TSH secretes T4 and T3. T4 is highest in populaton.; whereas, T3 is little. However, T4 converts into T3. T3 itself is more potent; meaning active compare to T4. Therefore, TRH and TSH are regulated by T3.

When T3 is low, TSH will secrete T4 and T3. T4 will convert to T3. T3 will raise up to norma levelsl.

When T3 is high, it will inhibit TSH. TSH will not secrete T4 and T3. T4 will not convert to T3. T3 levels will drop down to normal levels.
You're abseloutely right about T4 being converted to T3 peripherally, however, that does not change the fact that T4 is the main feedback regulator in this system. So an increase in T4 would go to the pituitary and get converted to T3 INSIDE the pituitary (it has the 5' deiodeinase enzyme). So if you wanted to DECREASE your T4, you inject the patient with T4. TSH is mostly responsive to T4 levels, not T3.
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Old 01-16-2013
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Quote:
Originally Posted by SarahM View Post
You're abseloutely right about T4 being converted to T3 peripherally, however, that does not change the fact that T4 is the main feedback regulator in this system. So an increase in T4 would go to the pituitary and get converted to T3 INSIDE the pituitary (it has the 5' deiodeinase enzyme). So if you wanted to DECREASE your T4, you inject the patient with T4. TSH is mostly responsive to T4 levels, not T3.
Can you give the source of who told you TSH is most responsive to T4? I never learned it as such.

Regardless, T3 is still the best answer, even if it has a smaller role. Reverse T4 is not a thing, T4 would increase T4 levels...
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Old 01-16-2013
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Quote:
Originally Posted by Avicenna04 View Post
Can you give the source of who told you TSH is most responsive to T4? I never learned it as such.

Regardless, T3 is still the best answer, even if it has a smaller role. Reverse T4 is not a thing, T4 would increase T4 levels...

T4 is more protein bound, so it has a higher plasma half life, thats why many books argue that it is responsible for most negative feedback. T3 is very biologically active, not highly bound to proteins and has a very high affinity to receptors so it gets taken up by tissue right away (I believe the t1/2 is less than a day).
Now about references, I've learned this in medical school which means Guyton but i'm not going to refer you there because no one has time for it. Kaplan talks about this concept also (2012 edition page 333-332 under Control of Thyroid Hormone Secretion). I'm going to quote what they said in their last sentence:

"As long as circulating free T4 remains normal, changes in circulating T3 have minimal effects on TSH secretion. However, TSH secretion increases if there is a significant drop in circulating free T4, even in the presence of an increase in circulating T3." (Kaplan, 2012)
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Quote:
Originally Posted by SarahM View Post
T4 is more protein bound, so it has a higher plasma half life, thats why many books argue that it is responsible for most negative feedback. T3 is very biologically active, not highly bound to proteins and has a very high affinity to receptors so it gets taken up by tissue right away (I believe the t1/2 is less than a day).
Now about references, I've learned this in medical school which means Guyton but i'm not going to refer you there because no one has time for it. Kaplan talks about this concept also (2012 edition page 333-332 under Control of Thyroid Hormone Secretion). I'm going to quote what they said in their last sentence:

"As long as circulating free T4 remains normal, changes in circulating T3 have minimal effects on TSH secretion. However, TSH secretion increases if there is a significant drop in circulating free T4, even in the presence of an increase in circulating T3." (Kaplan, 2012)

Cool, good catch! But I guess T3 is still the only real answer here. But it is obviously not the best question.

Thanks for the clear up
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  #19  
Old 01-25-2013
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it's gotta be T3...
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Old 05-13-2013
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it should be t4 b/c it is the main regulatory factor to which tsh is concerned though its acts by converting into t3 ,
but wrt the bcq iil go with t4
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Old 05-13-2013
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I would go for A or D
Cannot be T3 or T4!
Injecting T4 would increase total T4.
Injecting T3 cannot have any effect in negative feedback through TSH on T4. T4 is the only form needed to have negative feedback effect on TSH according to Kaplan. Remember that T3 only plays a role only in the thyrotropes in the anterior pituitary after T4 is converted to T3 in the thyrotropes. But circulating T3 has no negative feedback mechanism on TSH.
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