Exogenous T3 administration... effect on T4?

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So UWorld says that a person taking exogenous T3 would have NO effect/change in T4 or rT3 levels.

Specifically, the lab values are supposed to be:
(-) TSH, no change T4, (+) T3, no change rT3

-----------------------------------------------------

Now, I don't understand why T4 is unchanged. I would think it would be DECREASED.

I get that taking T3 would feedback (-) TSH. I also get the "learning pt" of the Q that T4 is converted to T3 & rT3... but rT3 can't be made fr T3. So, it's true that an increase in T3 would not directly affect levels of rT3 or T4.

BUT, what I don't get is why, once TSH is decreased, there wouldn't be a subsequent drop in T4 (as well as rT3). Since there's less TSH stimulation, wouldn't there be less synthesis of T4?

Anyone? 😕

 

[Step1Hash]

So UWorld says that a person taking exogenous T3 would have NO effect/change in T4 or rT3 levels.

Specifically, the lab values are supposed to be:
(-) TSH, no change T4, (+) T3, no change rT3

-----------------------------------------------------

Now, I don't understand why T4 is unchanged. I would think it would be DECREASED.

I get that taking T3 would feedback (-) TSH. I also get the "learning pt" of the Q that T4 is converted to T3 & rT3... but rT3 can't be made fr T3. So, it's true that an increase in T3 would not directly affect levels of rT3 or T4.

BUT, what I don't get is why, once TSH is decreased, there wouldn't be a subsequent drop in T4 (as well as rT3). Since there's less TSH stimulation, wouldn't there be less synthesis of T4?

Anyone? 😕

what is the question ID#?

 

[unsung]

what is the question id#?

1615.

 

[kstreet]

In the question the person is already hypothyroid , very high TSH levels and has low T4 levels. I guess you can't get much lower than low because the thyroid ain't responding to TSH. In any case none of the answer choices say "Decreased" for T4 so the only one that works is the "correct" answer.

 

[Step1Hash]

In the question the person is already hypothyroid , very high TSH levels and has low T4 levels. I guess you can't get much lower than low because the thyroid ain't responding to TSH. In any case none of the answer choices say "Decreased" for T4 so the only one that works is the "correct" answer.

Agreed. Changes in TSH wont really affect someone with hashimotos.

 

[kaleerkalut]

So UWorld says that a person taking exogenous T3 would have NO effect/change in T4 or rT3 levels.

Specifically, the lab values are supposed to be:
(-) TSH, no change T4, (+) T3, no change rT3

-----------------------------------------------------

Now, I don't understand why T4 is unchanged. I would think it would be DECREASED.

I get that taking T3 would feedback (-) TSH. I also get the "learning pt" of the Q that T4 is converted to T3 & rT3... but rT3 can't be made fr T3. So, it's true that an increase in T3 would not directly affect levels of rT3 or T4.

BUT, what I don't get is why, once TSH is decreased, there wouldn't be a subsequent drop in T4 (as well as rT3). Since there's less TSH stimulation, wouldn't there be less synthesis of T4?

Anyone? 😕

Just stepped into the thread and I have not reviewed this material but I did want to say that I remember that most T4 comes from peripheral conversion of T3 so that might be offsetting the decrease from TSH stimulation.

Just read the other post now and being hypothyroid, this also would make sense that it wouldn't go lower.

Hope this helps 🙂

 

[kstreet]

Just stepped into the thread and I have not reviewed this material but I did want to say that I remember that most T4 comes from peripheral conversion of T3 so that might be offsetting the decrease from TSH stimulation.

Just read the other post now and being hypothyroid, this also would make sense that it wouldn't go lower.

Hope this helps 🙂

T4 is peripherally converted into T3 and Reverse T3

 

[Step1Hash]

Just stepped into the thread and I have not reviewed this material but I did want to say that I remember that most T4 comes from peripheral conversion of T3 so that might be offsetting the decrease from TSH stimulation.

Just read the other post now and being hypothyroid, this also would make sense that it wouldn't go lower.

Hope this helps 🙂

I think you have it backwards.... the thyroid mainly makes T4 which gets peripherally converted to T3/rT3 by 5'iodinase. But it's good that you remember there is peripheral conversion in general having not looked at endocrine yet.