Eu-thyroid Sick Syndrome Confusion

[masterofnone101]

So i understand that it primarly causes Low T3, Normal T4 and TSH ... but im curious to know what is the reason it is at a euthyroid state. Wouldn't the low t3 feedback to the TSH and attempt to produce T4 which would be converted to T3?

And when the word euthyroid is implied does that refer to TSH only being normal regardless of the T4/T3 concentration?

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[Phloston]

So i understand that it primarly causes Low T3, Normal T4 and TSH ... but im curious to know what is the reason it is at a euthyroid state. Wouldn't the low t3 feedback to the TSH and attempt to produce T4 which would be converted to T3?

And when the word euthyroid is implied does that refer to TSH only being normal regardless of the T4/T3 concentration?

Funny the timing of this question because I'm on an endocrine rotation right now (yes an actual rotation just for endocrine).

Euthyroid sick syndrome is caused by illness, trauma, surgery, or any inciting stress factor that leads to a decreased conversion of T4 to T3.

So TSH: normal; T4: normal; T3, low (in reality, and according to MKSAP, TSH can be high, low or normal, but classic sick syndrome is normal TSH).

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I've seen two cases so far in real life:

First one consultant showed me an elderly woman's TFTs over a one-year period. Everything was normal for 4 months, followed by an 8-month period where only T3 was low but gradually improving. Consultant's question was 'do you want to ask what happened 8 months ago?' She said, 'well she fell and fractured her neck of femur.' So I said, 'so...she had surgery." Consultant says 'yes? so what's the picture then?'

Second one was a younger patient who just had a thyroidectomy and was now on thyroxine (exogenous T4). TSH was low, T4 normal, and T3 low. The team was a bit confused, but my impression (whether right or wrong) was euthyroid sick syndrome merely with low TSH (as this is possible as a simple explanation although not classic), or euthyroid sick syndrome + low TSH in the context of long-term anterior pituitary thyrotrophic suppression from the prior hyperthyroidism (and would take a little time to start producing TSH). One could also argue TSH would be suppressed if too much thyroxine were given, but in that context T4 would be elevated, especially since in sick syndrome there's decreased conversion.

 

[tasar1898]

@Phloston So the mechanism in the classic form is that catabolic states produce epi/cortisol that reduce the activity of the 5-iodenase-something enzyme so we get less T3 while T4/TSH loop keep working normally??

 

[Phloston]

@Phloston So the mechanism in the classic form is that catabolic states produce epi/cortisol that reduce the activity of the 5-iodenase-something enzyme so we get less T3 while T4/TSH loop keep working normally??

HY for the USMLE is that propranolol (and probably most non-selective beta-blockers) decreases conversion of T4 to T3, so we can infer beta agonism probably increases conversion. Any catecholamines floating around in a state of stress (NE for beta-1; E for beta-1 and -2) would probably therefore increase conversion. I'm not sure about cortisol specifically but I wouldn't doubt it.

 

[tasar1898]

Yeah propranolol , how couldnt I remember.. Funny how you forget most stuff after finishing step 1. My guess is ''chronic'' elevated cortisol downregulates the enzyme to reduce energy expenditure in the peripheral cells . Makes sense to conserve proteins/sugar for APR's and basic functions

 

[Transposony]

IIRC Euthyroid sick syndrome is most commonly seen in ICU.

Possible causes:
Cytokines
Deiodination
Inhibition of TRH and TSH secretion
Inhibition of plasma membrane transport of iodothyronines
Thyroid-binding globulin decrease/desialation
Reduction in TR expression as well as DNA binding
Drugs that influence thyroid function