Thyroid function in critical illness

Abstract

Themetabolic support of a critically ill patient is a relatively new topic of active research and discussion, and little is known about the effects of critical illness on metabolic physiology and activity. The nonthyroidal illness syndrome, also known as the low T3 syndrome or euthyroid sick syndrome, is characterized by abnormal thyroid function tests encountered in patients with acute or chronic systemic illnesses. The laboratory parameters of this syndrome include low serum levels of triiodothyronine (T3) and high levels of reverse T3, with normal or low levels of thyroxine (T4) and normal or low levels of thyroid-stimulating hormone (TSH). This condition may affect 70 % of critically ill patients.The changes in serum thyroid hormone levels in the critically ill patient are the result of alterations in TSH regulation, in the peripheral metabolism of the thyroid hormones, in the binding of thyroid hormone to transport protein, in receptor binding, and in intracellular uptake.Medications have also a very important role in these mechanisms. Hormonal changes can be seen within the first hours of critical illness, and interestingly, these changes correlate with the final outcome. Data of a beneficial effect on outcome with thyroid hormone treatment in critically ill patient are so far controversial.Thyroid function generally returns to normal in survivors as the acute illness resolves.