Euthyroid sick syndrome(ESS) is a condition where the thyroid function is abnormal because of the systemic disease outside the thyroid gland, surgery, or fasting, etc. ESS can be seen in all ages, but is most common in the elderly, and the most common type is low-T3 syndrome. Mechanism of low-T3 syndrome includes bodily "protective" defense, selenium deficiency, hypothalamus-pituitary-thyroid axis abnormalities, thyroid hormone binding globulin (TBG) changes, and so on. The risk of low-T3 syndrome significantly increases in the elderly with diabetes, heart failure, chronic respiratory diseases and surgery, etc. These geriatric diseases complicated with low-T3 syndrome suggest more serious condition and worse prognosis, so it is very important to have an accurate judgement for the thyroid function. Whether low-T3 syndrome needs treatment or not is still highly controversial.