euthyroid sick syndrome is sometimes called "low T3 syndrome." Also you know that the patient is euthyroid because her T4 and TSH are within the reference range. She is sick.

I am for some reason learning about this for the first time so I'll write a bit about euthyroid sick syndrome in case it helps someone.

It's a disease of exclusion - the thyroid hormones are whack but the thyroid gland itself seems to be totally fine. Specifically, T3 levels are low, rT3 is high (the activity of different deiodinases are off, so rT3 is made more and degraded less, though it doesn't seem like the rT3 level has any other clinical implications). As the other comments have mentioned, TSH and T4 are typically normal, although they can be decreased as well in severe cases.

Euthyroid sick syndrome is typically due to an underlying critical illness or starvation - the body stops making the hormones as an extreme measure to save energy and resources. The treatment is to treat the underlying illness; thyroid hormone therapy is not recommended as its effectiveness is inconclusive.

In this question stem, the patient has an underlying illness. Her thyroid gland works fine (responds appropriately to TSH). Classic case of euthyroid sick syndrome.

Given that her T4 levels are normal, we can rule out B, D, and E. I'm a dumdum and put C because I had no idea what euthyroid sick syndrome was. But given that her TSH level is also normal, it's not an issue with the hypothalamus or pituitary gland.

(Sources: wiki page and this article)

Euthyroid sick syndrome = levels of T3 and/or T4 are abnormal, but the thyroid gland does not appear to be dysfunctional. The classical phenotype of this condition is often seen in starvation, critical illness, or patients in the intensive care unit. The most common hormone pattern is low total and free T3, elevated rT3, and normal T4 and TSH levels.

In euthyroid sick syndrome, there will be normal rT3, low T3, and normal TSH → this is because in critical illness there is decreased activity of D1/D2 5-monodeiodinase activity (normally T4 → T3, and rT3 → T2), and increased activity of D3 5' monodeiodinase (normally T4 → rT3)

Patients with severe Euthyroid Sick Syndrome may have low free T4, low total T4, AND low TSH levels

Euthyroid sick syndrome

Due to: severe systemic illness (sepsis)

Path: cytokines (IL-1, TNFa, IL-6) --> decreased peripheral 5'-deiodinase *

Labs: normal TSH, normal T4, low T3, high rT3 (recall that T4 can be converted to either T3 or rT3).

*there are many mechanisms that lead to this; however, this is the most clinically relavent for labs