The functioning of the thyroid gland is of great importance for the formation, maturation and activity of the central nervous system. The association of clinical thyroid dysfunction with mental disorders, including cognitive impairment, seems to be well documented. Abnormal concentrations of thyroid hormones can lead to deterioration of cognitive processes through changes in neurotransmission, intensification of oxidative stress, or impact on β-amyloid transformation and glucose metabolism in the central nervous system. Doubts concern mainly subclinical forms of thyroid dysfunction. According to some data, they are supposed to be related to the state of cognitive functions and to be one of the factors accelerating the mechanisms leading to degeneration of the brain tissue and, consequently, development of dementia. The results of studies on the correlation of thyroid activity with cognitive functions and the possible beneficial effects of hormonal supplementation on cognitive processes, however, bring contradictory results, which may be at least partly due to large methodological problems. One should also not exclude a reverse correlation, where the ongoing neurodegenerative process would affect thyroid function, e.g., by the changed production and secretion of thyroliberin. Despite several decades of intensive research, the explanation of this relationship is still far from conclusive