As a general rule level of rT3 always parallel to T4. If it T4 goes up rT3 will goes up.
In central hypothyroidism T4 decrease and rT3 will decrease also.
In any acute illnesses or critically ill pt, there is decrease in TSH and low T4/T3.
rT3 will increase due to impaired clearance.
This us called Euthyroid sick syndrome.
So in any critically ill pt if you see decrese in TSH,T4,T3
Go and check rT3 if its low so this is a central hypothyroidism. If it’s high this is sick thyroid syndrome.