For more than 3 decades it has been known that serum thyroid hormone levels drop during starvation and illness. In mild illness, this involves only a decrease in serum T3 levels. However, as the severity of the illness increases, there is a drop in both serum T3 and T4 (1). This decrease in serum thyroid hormone levels is seen in starvation (2), sepsis (3, 4), surgery (5), myocardial infarction (6, 7), bypass (8), bone marrow transplantation (9), and, in fact, probably any severe illness. Based on the conviction that patients with these abnormalities are not hypothyroid despite the low hormone levels in blood, the condition has been called the euthyroid sick syndrome. An alternative designation, which does not presume the metabolic status of the patient, is nonthyroidal illness syndrome (NTIS). NTIS seems a preferable name in light of present knowledge and will be used in this review.