THE TERM euthyroid sick syndrome (ESS) identifies abnormalities in thyroid function tests observed in patients with systemic nonthyroidal illnesses (NTIs) and those undergoing surgery or fasting (1, 2). The term nonthyroidal illness syndrome (NTIS) has also been employed to describe these abnormalities (3). These abnormalities result from variable, usually reversible, disturbances in the hypothalamopituitary-thyroid axis, thyroid hormone binding to serum proteins, tissue uptake of thyroid hormones, and/or thyroid hormone metabolism. Several recent reviews have addressed these issues (3–6). I shall focus mainly on the clinical diagnosis, significance, and treatment of ESS.