Clinical Significance of Low-Triiodothyronine Syndrome in Patients Requiring Non-Surgical Intensive Care　― Triiodothyronine Is a Comprehensive Prognostic Marker for Critical Patients With Cardiovascular Disease ―

Background: Low-triiodothyronine (T(3)) syndrome is a known complication in intensive care unit (ICU) patients, but the underlying mechanisms and prognostic impact are unclear. Methods and Results: This study retrospectively enrolled 2,976 patients who required care in the ICU. Of these patients, 2,425 were euthyroid and were divided into normal (n=1,666; free T(3) [FT(3)] ≥1.88 µIU/L) and low-FT(3) (n=759; FT(3) <1.88 µIU/L) groups. Multivariate logistic regression analysis revealed that prognostic nutritional index >46.03 (odds ratio [OR] 2.392; 95% confidence interval [CI] 1.904–3.005), age (per 1-year increase; OR 1.022; 95% CI 1.013–1.031), creatinine (per 0.1-mg/dL increase; OR 1.019; 95% CI 1.014–1.024), and C-reactive protein (per 1-mg/dL increase; OR 1.123; 95% CI 1.095–1.151) were independently associated with low FT(3). Survival rates (within 365 days) were significantly lower in the low-FT(3) group. A multivariate Cox regression model showed that low FT(3) was an independent predictor of 365-day mortality (hazard ratio 1.785; 95% CI 1.387–2.297). Low-T(3) syndrome was significantly more frequent in patients with non-cardiovascular than cardiovascular diseases (73.5% vs. 25.8%). Prognosis was significantly poorer in the low-FT(3) than normal group for patients with cardiovascular disease, particularly those with acute coronary syndrome and acute heart failure. Conclusions: Low-T(3) syndrome was associated with aging, inflammatory reaction, malnutrition, and renal insufficiency and could lead to adverse outcomes in patients admitted to a non-surgical ICU.