Early hypothyroidism produces a generalized damage in the brain and in particular, changes in the connectivity of neocortical sensory areas. In this paper, the potential alterations in local neocortical circuits have been explored using immunocytochemistry for parvalbumin (PV) in normal and hypothyroid adult rats. The number and radial distribution of PV-positive cells were similar in both groups, but in hypothyroid rats, the density of PV-positive terminal-like puncta and processes was dramatically reduced, especially in layers II-III and the lower part of layers IV-V and VI. These results suggest that thyroid hormones are necessary for normal development of cortical circuits in which PV-positive cells are involved.