This study investigated the cognitive sphere in young and middle-aged patients with hypothyroidism. The aim of the study was to determine the dependence of cognitive impairment on the severity of hypothyroidism and the duration of the disease history. Despite advances in understanding the metabolism and the impact of thyroid hormone on the human brain, the scientific debate over the reversibility of neuropsychiatric disorders in hypothyroidism continues. There have been many studies about the relationship between neuropsychiatric disorders and hypothyroidism among newborns, children, the elderly, as well as in conjunction with other pathologies such as stroke, myocardial infarction, chronic cerebral ischemia, metabolic syndrome, etc. However, a neuropsychological study in young and middle age period patients suffering hypothyroidism, with the exclusion of another pathology, was not carried out. The neuropsychological examination was carried out in 61 patients with clinical and biochemical signs of hypothyroidism before and after replacement therapy. Patients were divided into test groups according to the severity of hypothyroidism and the duration of the history. The research findings showed the prevalence of cognitive impairments in basic groups of patients compared with the test ones. It is interesting, that different types of cognitive functions are affected in varying degrees. The findings can be used in endocrinology, neurology, therapy, practice of family doctors, in teaching university- and master course students as well as physicians . It has been established that disorders of higher cortical functions are directly proportional to the severity of hypothyroidism and the duration of illness; moreover, replacement therapy is not effective enough to restore cognitive function.