Acute pancreatitis (AP) patients commonly develop nonthyroidal illness syndrome (NTIS) within 7 days after disease onset. NTIS was found to be linked with worse characteristics at admission and poor outcome during the course. Free triiodothyronine (FT3) merits exploration as a potential biomarker in the prediction of death in AP cases.

• This retrospective review of AP patients with a diagnosis of NTIS from Jan 2012 to September 2020 was conducted to determine the prevalence, characteristics, and prognosis of NTIS and its correlation with outcomes in AP patients.

• During a period of 8 years, NTIS diagnosis was made in 183 included AP patients, representing an incidence of 64.7%.

• Relative to euthyroid patients, NTIS patients were admitted with worse condition based on the higher Acute physiology and chronic health evaluation II score, Sequential organ failure assessment score, Balthazar's CT score, C-reactive protein and lower albumin.

• A longer ICU duration and more probability of developing infected pancreatic necrosis (15.3% vs 6.3%) and gastrointestinal fistula (6% vs 0%) were noted in NTIS patients vs euthyroid patients.

• For predicting death, the performance of FT3 was the best, vs other well-recognized biomarkers.