Thyroid hormones (THs), T3 and T4, have many physiological actions and are essential for normal behavioral, intellectual and neurological development. THs have a broad spectrum of effects on the developing brain and mediate important effects within the CNS throughout life. Insufficient maternal iodine intake during gestation and TH deficiency during human development are associated to pathological alterations such as cretinism and mental retardation. In adulthood, thyroid dysfunction is related to neurological and behavioral abnormalities, including memory impairment. Analysis of different experimental models suggests that most of the effects on cognition as a result of thyroid dysfunction rely on hippocampal modifications. Insufficiency of THs during development thus alters hippocampal synaptic function and impairs behavioral performance of hippocampal-dependent learning and memory tasks that persist in euthyroid adult animals. In the present review, we summarize the current knowledge obtained by clinical observations and experimental models that shows the importance of THs in learning and mnemonic processes.