Aneurysmal subarachnoid hemorrhage (aSAH) is a recently identified risk factor for chronic hypothyroidism. Patients with hypothyroidism often exhibit cognitive dysfunction. The aim of the present study was to determine the effects of thyroid hormone replacement therapy on cognition in aSAH survivors with hypothyroidism. A study population of 135 patients was recruited and subjected to the Montreal Cognitive Assessment (MoCA) and Beck Depression Inventory. Among the study population, 52 patients exhibited cognitive dysfunction. Thyroid hormone levels were measured in these patients using an electrochemiluminescence immunoassay in order to elucidate possible deficits in the thyrotrophic hormonal axes, and hypothyroidism was confirmed in 31 patients. Among these 31 patients, 22 patients consented to be randomized into groups and were administered levothyroxine replacement or a placebo treatment for 3 months. The MoCA and Wechsler Adult Intelligence Scale-Chinese version (WAIS-RC) testing were performed prior to and following the replacement therapy or placebo treatments. All subjects completed the study with no negative side effects. After 8-12 weeks of oral levothyroxine administration, it was observed that the serum concentration of thyroid-stimulating hormone was restored to normal levels. Furthermore, neuropsychological test results improved following the replacement therapy. A significant improvement was observed in the MoCA scores of the replacement group following therapy, with the exception of the score for abstraction. Additionally, significant improvements in the WAIS-RC were observed in the replacement group, with the exceptions of the information comprehension and letter-number sequencing scores. Thus, the present study has demonstrated the partial normalization of cognitive impairments in patients with hypothyroidism following aSAH as a result of appropriate levothyroxine replacement therapy.