Effects of illness on thyroid function tests

Severe illness of any type predictably leads to abnormal results on thyroid function tests. The first apparent changes are a decrease in total triiodothyronine (T3) and an increase in reverse T3. As disease progresses, a marked decline in thyroxine and an increase in T3 resin uptake are observed. Thyrotropin levels remain normal, confirming that the patient is euthyroid despite marked alterations in thyroid function tests. Supplemental thyroid hormone has never been shown to be of any value, and may indeed be harmful, in euthyroid patients with abnormal thyroid function. Recognition of the euthyroid sick syndrome is vital to avoid needless therapy.