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Nonthyroidal Illness Syndrome

Meaning

Nonthyroidal Illness Syndrome (NTIS), also known as Euthyroid Sick Syndrome, is a clinical condition characterized by abnormal thyroid hormone levels—typically low triiodothyronine (T3) and sometimes low thyroxine (T4) or elevated reverse T3 (rT3)—in the presence of a non-thyroidal systemic illness, trauma, or caloric deprivation, without primary thyroid gland dysfunction. This is a crucial endocrine adaptation, representing a systemic metabolic down-regulation intended to conserve energy during acute stress or critical illness. The syndrome is an important diagnostic consideration, as it must be differentiated from true hypothyroidism.