Cognitive function and hypothyroidism are strongly associated, and age-related differences in performance are significant. The association between hypothyroidism and cognitive impairment in adult patients is still debatable, although it is most severe in the fetus and neonatal period, where it is easy to leave lasting sequelae. Current theories postulate that the various age-related manifestations of this cognitive impairment may be linked to the various stages of hypothyroidism during the time-dependent development of the neurological system. Although the precise mechanism is still not entirely understood, it might be connected to immunological factors.