Beyond Euthyroid Sick Syndrome: Pre-Admission Levothyroxine Use and Thyroid Hormone–Outcome Associations in Hospitalized Patients

Background/Objectives: Pre-admission levothyroxine use may influence the interpretation of thyroid tests beyond the conventional framework of non-thyroidal illness syndrome (NTIS). We examined whether thyroid hormone–outcome associations differed by treatment status.

Methods

This retrospective study included medical inpatients with thyroid-stimulating hormone (TSH) < 5 mIU/L during 2006–2024. Propensity-score matching used age, sex, and 16 comorbidity indicators. Complete-case negative-binomial and logistic models examined hospitalization length and in-hospital mortality, with treatment-by-hormone interactions.