Effects of thyroid hormones and their metabolites on learning and memory in normal and pathological conditions.

Background: Classical thyroid hormones have an established necessary role in the normal development of the central nervous system, and they have been recently considered as decisive factors influencing cognitive functions in the adult brain and involved in the development of Alzheimer's disease. The picture summarizing thyroid hormone effects on the adult brain, however, does not only include classical thyroid hormones but also the products of their peripheral metabolism. These latter have been considered as inactive breakdown products for long but recently were proved to produce significant biological effects. Objective: In this review article we presented recent evidence supporting the hypothesis that thyroid hormones exert a neuroprotective effect in the brain areas involved in learning and memory. Moreover, we summarized the evidence that suggests that non-classical thyroid hormones produce significant neurological effects in the adult brain. We also discussed the possible role of thyroid hormones in the cognitive impairment, typical of Alzheimer's disease. Methods: A comprehensive review of the literature based on the current knowledge of the effects of classical and non-classical thyroid hormones on the adult brain and their role in Alzheimer's disease was performed. Results: The available literature suggests that both classical and non-classical thyroid hormones act as neuroprotective agents in the brain areas related to learning and memory. Their role in these areas supports the idea that they may be involved in the development of Alzheimer's disease. Conclusion: Thyroid hormones produce significant neurological effects, act as neuroprotective agents and might be considered as future diagnostic and therapeutic tools for Alzheimer's disease.