Blood thyroid function tests (TFT) are routinely used to screen for thyroid disorders in several clinical settings. TFT on hospital admission may also be useful prognostic predictors of acute IS: according to recent evidence, poststroke outcome is better in patients with lower thyroid function and worse in those with higher thyroid function. However, previous reports are few and mostly compared patients with thyroid disorders to euthyroid patients. Thyroid disorders are known risk factors for cerebrovascular disease. However, hyperthyroidism is related to cardioembolic IS whereas hypothyroidism is related to atherosclerotic risk factors. Therefore, findings from available studies of TFT might just reflect the worse prognosis of cardioembolic IS compared to other IS subtypes. Another limitation of previous studies is the lack of information for older persons, who represent three quarters of all IS patients. In this paper, we investigated whether serum thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) measured on Stroke Unit (SU) admission are associated with early outcomes of acute IS in 775 euthyroid patients aged ≥65 years (mean age 80.1 ± 8.7 years). Two composite outcomes were investigated: poor functional outcome (death during SU stay or disability at SU discharge), and unfavorable discharge setting (death during SU stay, transfer from SU to other acute hospital unit or transfer from SU to long-term care-facilities as opposed to direct discharge home). Analyses were performed using logistic regression models. Curvilinear associations were tested including TFT as polynomial terms. Models were adjusted for demographics, prestroke, and IS-related confounders. We found that lower TSH had a complex curvilinear association with poor functional outcome and that the shape of the associations changed with age. At age 65, the curve was U-shaped: outcome risk decreased with increasing TSH, reached its minimum at TSH near 3.00 mUI/L and then started to rise. Between ages 70 and 75, however, the shape of the curve straightened and, starting from age 80 took an inverted U-shape: outcome risk rose with increasing TSH, reached its maximum at TSH values that progressively shifted upward with increasing age (from 1.70 mU/L at age 80 to about 2.20 mUI/L at age 90), then started to decrease. A linear inverse association was found between FT3 and unfavorable discharge setting. Our study suggests that measurement of TFT on SU admission can provide independent prognostic information for early outcomes of acute IS in older euthyroid patients.