Thyroid hormone physiology changes during critical illness. Circulating concentration of triiodothyronine (T3), the active form of thyroid hormone decreases. It has long been uncertain whether this represents a pathologic change or if it is an adaptive phenomenon. Controlled clinical trials have been required to understand whether replacing and restoring serum T3 levels is therapeutic.

Clinical trials of T3 have recently been proposed with some completed. These have been conducted in patients with sepsis, myocardial infarction, infants undergoing cardiac surgery, and acute respiratory distress syndrome. Of the completed trials, T3 administration restored serum concentrations, but was not accompanied by significant clinical benefit. Importantly, restoring serum T3 levels did not cause any adverse effects.

If T3 is to be considered a therapeutic target in critical illness, further studies should consider the stage of disease it is administered, and whether there are other surrogate measures to assess adequacy of hormone replacement over and above serum T3 concentrations.