Thyroidal Changes During Critical Illness

Patients suffering from a variety of critical illnesses present with uniform alterations within the thyroid axis with low plasma triiodothyronine (T3), but increased plasma reverse T3 (rT3). As these changes occur in the presence of low-normal thyroid stimulating hormone (TSH), this constellation is also referred to as Nonthyroidal Illness Syndrome (NTI). Both central and peripheral components of the thyroidal axis play a role in the development of NTI. Furthermore, nutritional intake can affect the extent and composition of NTI. The severity of NTI is associated with a poor prognosis, but it is still unclear whether this indicates a causal relationship, or in contrast, an adaptation to more severe illness.