Abstract
Nonthyroidal illness syndrome (NTIS) is a common finding in critically ill patients, characterized by disruptions in the hypothalamus-pituitary-thyroid axis, resulting in altered levels of thyroxine (T4), triiodothyronine (T3), and reverse T3. This condition, often considered to be an adaptive response aimed at conserving energy, can become maladaptive in prolonged critical illness, contributing to poor outcomes in intensive care unit patients. The pathophysiology of NTIS involves cytokine-driven alterations in thyroid hormone (TH) metabolism, impaired hormone transport, and reduced receptor sensitivity, which-collectively-suppress thyroid function. Despite these insights, the therapeutic role of TH replacement in patients with NTIS remains uncertain. Low doses of levothyroxine and T3 have been trialed, particularly in patients with cardiovascular comorbidities, but clinical studies report conflicting results regarding their impact on mortality and overall patient outcomes. While some evidence suggests potential benefits of T3 administration in specific subgroups, such as patients with septic shock or severe coronavirus disease 2019, robust clinical trials have yet to conclusively demonstrate improved survival or recovery. The heterogeneity in NTIS presentation and treatment protocols, as well as the complex nature of TH regulation in critically ill patients, complicates efforts to establish clear guidelines for hormone therapy. Future research should prioritize individualized approaches, optimizing hormone dosing and timing, while aiming to elucidate the long-term effects of such interventions on critically ill patients to improve morbidity and mortality outcomes.