Euthyroid sick syndrome is not very well understood.
It is a condition where TSH and T3/T4 levels are low, but there is no underlying pathology of the thyroid. These test results are the same as those found in secondary hypothyroidism (i.e. usually caused by a pituitary problem), and as a result, differentiation is difficult.
It is associated with stress, and thus if serum cortisol levels are raised, it is likely that euthyroid sick syndrome is present as opposed to a pituitary problem.
If serum cortisol and gonadotropin levels are low, then secondary hypothyroidism should be suspected.
It often presents in association with an acute illness, and as a result it may be useful to delay diagnosis until the acute episode has passed, as this acute episode could affect thyroid function test results.