It is well known that thyroid hormone plays a crucial role in the development and maturation of the nervous system. However, little is known about the role of thyroid hormone in the adult brain. In this short review we have dwelt on this point, with regard to the role of thyroid hormone on neuropeptide gene expression regulation in the paraventricular nucleus of the hypothalamus and in extrahypothalamic brain areas, on neurotrophin and neurotrophin receptor expression in the hippocampus and basal forebrain in basal conditions, and after neurotoxic challenges. Effects of hypothyroidism are discussed in view of a possible role of thyroid status in brain aging quality.