What is non-thyroidal illness or sick euthyroid syndrome?
The simplest way to explain is that when the body in under tremendous stress, it reduces the amount of thyroid hormone produced. Typically, under these conditions thyroid hormone replacement is not provided and once the stress is gone, the thyroid hormone production rebounds back to normal.
This can be seen is children who are in the intensive care unit or even those who had a recent viral infection. It is important to repeat blood work in these cases once the illness is resolved to show that the thyroid hormone production has normalized.