Hypo- and hyperthyroidism affect NEI concentration in discrete brain areas of adult male rats

► Thyroid hormones modify NEI content in specific areas of the CNS related to neuroendocrine control and reproduction. ► Long-term hypothyroidism (24 days) induces reductions in NEI concentration, in the PeFLH and PVH in the morning and afternoon, in the OVLT + AVPV and ME + Arc only in the morning, and in the POA just in the afternoon. ► NEI content changes during hyperthyroidism, principally in POA at morning and in ME + Arc and Pi at afternoon. ► NEI concentration changes in specific brain areas secondary to hypo- and hyperthyroidism may have consequences itself on reproduction.