Low fT3 Syndrome, Dialysis Inadequacy, and Death Occurrence in Hemodialysis Patients: Evidence of a Vicious Circle from a Prospective Bi-Center Observational Study

<b>Background/Objectives:</b> Non-thyroidal illness syndrome (NTIS) also known as low FT3 syndrome is characterized by altered thyroid hormone levels during severe illness, is common in end-stage renal disease, and reflects metabolic and inflammatory stress. This study evaluated the thyroid hormone profiles of patients undergoing maintenance hemodialysis, assessing relationships between NTIS severity and dialysis adequacy while accounting for mineral and bone metabolism markers, anemia status, duration of dialysis therapy, and their association with the number of deaths during follow-up. <b>Methods</b>: This prospective bi-center study included adults receiving maintenance hemodialysis for at least 3 months. Patients treated for thyroid disease or taking medications affecting the hypothalamus-pituitary-thyroid axis were excluded. Thyroid-stimulating hormone, free triiodothyronine (fT3), and free thyroxine (fT4) levels were measured, and dialysis adequacy was assessed using spKt/V. Patients were classified as euthyroid or having NTIS (stratified by severity), and associations between clinical characteristics and the number of deceased patients during a 6-month observation period were analyzed using receiver operating characteristic (ROC) curves to determine prognostic cut-off values for thyroid hormones. <b>Results</b>: Among 74 patients, 50% had NTIS and exhibited significantly lower dialysis adequacy than euthyroid individuals (median spKt/V 1.0 vs. 1.1; <i>p</i> = 0.03), with spKt/V declining as NTIS severity increased (stages I-III, <i>p</i> = 0.008). NTIS severity correlated with age and pulmonary comorbidities, while mineral and bone metabolism markers were comparable between the groups. During the 6-month follow-up, 23% of the patients died, exhibiting significantly lower fT3 and fT4 levels than survivors. ROC analysis identified clinically relevant fT3 and fT4 cut-off values that were associated with the number of deaths. <b>Conclusions</b>: NTIS in hemodialysis patients correlates with reduced dialysis adequacy and appears to be a prognostic factor for risk of death. NTIS severity correlated with declining spKt/V, potentially reflecting disease burden, and thyroid hormone assessment may provide prognostic information.