Sick Euthyroid Syndrome

Critical illness causes multiple alterations in thyroid hormone concentrations in patients who have no intrinsic thyroid disease. These effects are nonspecific, and they relate to the severity of illness. Because a wide variety of illnesses tend to result in the same changes in serum thyroid hormone levels, such alterations in thyroid hormone indexes has been termed the sick euthyroid syndrome. These changes are rarely isolated, and they are often associated with alterations in other endocrine systems. Similar changes in endocrine function has been shown experimentally by administration of cytokines from the interleukin and interferon families, as well as tumor necrosis factor-α. Thus, the sick euthyroid syndrome should not be viewed as an isolated pathological event, but as part of a coordinated systemic reaction to illness that involves both the immune and the endocrine systems. Recovery from the illness usually results in resolution of the alterations in thyroid hormone parameters. Supplemental thyroid hormone therapy in patients with the sick euthyroid syndrome is of no benefit and is not indicated.