BackgroundThyroid hormones are of fundamental importance for brain function. While low triiodothyronine levels during acute ischemic stroke (AIS) are associated with worse clinical outcomes, dynamics of thyroid function after AIS remains unknown. Thus, we longitudinally evaluated thyroid hormones after stroke and related them to stroke severity.MethodsWe prospectively traced thyroid stimulating hormone (TSH), free triiodothyronine (fT3), and free thyroxin (fT4) levels from the hyper-acute (within 24 h) to acute (3–5 days) and chronic (3–6 months) stages of ischemic stroke using a mixed regression model. Then, we analyzed whether stroke severity at presentation, expressed by National Institute of Health Stroke Scale (NIHSS), is associated with change in thyroid function.ResultsForty-five patients were evaluated in hyper-acute and acute stages, while 29 were followed through chronic stage. TSH levels decreased from hyper-acute (2.91 ± 0.65 μIU/mL) to acute (2.86 ± 0.46 μIU/mL) and chronic stages of stroke (1.93 ± 0.35 μIU/m,p= 0.95). fT3 levels decreased from hyper-acute (2.79 ± 0.09 pg/ml) to acute (2.37 ± 0.07 pg/ml) stages, but recovered in chronic stage (2.78 ± 0.10 pg/ml,p< 0.01). fT4 levels decreased from hyper-acute (1.64 ± 0.14 ng/dl) to acute (1.13 ± 0.03 ng/dl) stages, and increased in the chronic stage (1.16 ± 0.08 ng/dl,p= 0.02). One-unit increase in presenting NIHSS was associated with 0.04-unit decrease of fT3 from hyper-acute to the acute stage (p< 0.01).ConclusionThere is a transient decrease of thyroid hormones after ischemic stroke, possibly driven by stroke severity. Larger studies are needed to validate these findings. Correction of thyroid function in acute stroke may be investigated to improve stroke outcomes.