Objective: Euthyroid sick syndrome (ESS) manifests by the decreased level of serum free thyroid hormones and with the normal or decreased secretion of thyroid-stimulating hormone. The study aims to identify the prevalence of ESS in elderly patients with sepsis and evaluate its possible effect on prognosis and mortality.Material and Methods: Two hundred and three patients diagnosed with sepsis were prospectively evaluated. They were divided into two groups, a geriatric group (≥65 years), and a control group (<65 years). Patients with low free T3 (fT3) and/or free T4 (fT4) were considered to have euthyroid-sick syndrome. The demographic characteristics, comorbidities, laboratory results, hospitalization data, intensive care unit (ICU) duration, treatment outcomes, and mortality rates of the patients were recorded and compared.Euthyroid sick syndrome: prevalence and prognosis in sepsis İstanbul Tıp Fakültesi Dergisi • J Ist Faculty Med Conclusions: The higher prevalence of ESS (88.5%) in elderly patients with sepsis was clearly demonstrated. Additionally, approximately two-fold higher mortality and ICU stay risk was documented in the ESS (+) group. Thus, simply screening of ESS in septic elderly patients will certainly contribute to treatment decisions and early prediction of complicated clinical course and poor prognosis.