BACKGROUND: non-thyroidal illness syndrome (NTIS) has been associated with an adverse clinical outcome. OBJECTIVE: to evaluate the prevalence of NTIS, its impact on patients' survival and the possible pathogenic role of systemic inflammation. DESIGN:observational cross-sectional analysis. PARTICIPANTS AND SETTING:three hundred and one acutely ill older patients (156 women; median age 81 years, range 65-101) consecutively admitted to a primary care unit. METHODS: serum FT(3), FT(4) and thyrotropin levels as well as acute inflammation indexes were evaluated. RESULTS: the NTIS prevalence (specifically low T3 syndrome) was 31.9%. A significant association was found between NTIS and acute renal failure (P = 0.006), New York Heart Association classification (NYHA) IV heart failure (P = 0.003) and metastasised cancer disease (P = 0.0002). Serum FT(3) values correlated inversely with serum C-reactive protein (P < 0.0001), lactate dehydrogenase (P = 0.0004), fibrinogen (P = 0.03) and erythrocyte sedimentation rate (P < 0.0001) values, and progressively decreased with increasing tertiles of age (P = 0.0004). The mortality rate was significantly higher (P = 0.0002) among patients with low T3 syndrome, which emerged as the sole predictive factor of death (odds ratio 4.3; 95% confidence interval 1.7-10.5). CONCLUSIONS: low T3 syndrome is very common in the hospitalised older population, emerging as the most sensitive independent predictor of short-term survival. Serum FT(3) determination should be included in the assessment of short-term prognosis of acutely ill older patients.