OBJECTIVE: Thyroid function may be useful prognostic predictor of acute ischemic stroke. However, the relationship between thyroid function and stroke prognosis remains controversial. We aimed to explore the correlation between thyroid function at admission and 90-day functional outcome in patients with acute ischemic stroke. METHODS: Our data were collected from patients with AIS (acute ischemic stroke) registered in the Stroke Center of Jiangsu Provincial Hospital of Chinese Medicine from January 2021 to July 2021.The outcome was divided into good outcome as mRS (Modified Rankin Scale) score <3, poor outcome as mRS≥3 (including hemorrhage, recurrence, and death within 90 days after stroke).Univariate, multivariate logistic regression analysis, tertile analysis and subgroup analysis were used to evaluate the relationship between TSH (Thyroid-stimulating hormone), FT3 (Free Triiodothyronine), FT4 (Free thyroxine) and 90-day outcome. RESULTS: 699 patients with AIS were included in this study. In univariate analysis, FT3 was negatively correlated with poor outcome of AIS patients at 90-day, TSH was not statistically correlated with 90-day outcome. Multivariate analysis showed that FT3 was negatively correlated poor outcome of AIS patients at 90-day. After adjusting for potential confounders, TSH was negatively correlated with poor outcome. Participants were categorized based on the tertile cut-off points of FT3 and TSH. With the increase of TSH value, the incidence of poor outcomes in Q3 was 0.57 times higher than that of Q1. Similarly, with the increase of FT3 value, the incidence of poor outcomes in Q3 is 0.3 times than that of Q1. CONCLUSIONS: FT3 and TSH were negatively correlated with poor 90-day outcome in patients with AIS. Measurement of thyroid function on admission may provide independent prognostic information for 90-day outcome of AIS.