Congenital hypothyroidism delays the formation and retards the growth of the mouse primary somatic sensory cortex (S1).

Intrinsic neural factors are thought to regulate the growth and formation of specific regions of the neocortex. To determine whether factors extrinsic to the brain are also involved in shaping the cytoarchitecture of the mammalian neocortex, we evaluated the effects of thyroid hormone (TH) on the formation and postnatal growth of the mouse S1 postero-medial barrel subfield (PMBSF). Congenital deficiency of TH, induced by propylthiouracil administration from day 12 of gestation, did not disrupt S1 specification, because no alterations in barrel number or configuration were observed in congenital hypothyroid mice. Barrel formation, however, was delayed by 3 days. In control mice, barrels were first seen at postnatal day (PN) 4, whereas in congenital hypothyroid mice they appeared at PN7. TH deficiency led to reduced adult brain, cortical, and S1 barrel dimensions. Barrel size, however, was relatively more affected than brain and cortical size. Our observations indicate, therefore, that TH does not participate in S1 specification, but in timing its formation. Our findings also indicate that TH regulates the relative size of the S1 barrel field by modulating the developmental timing of areal specification and brain growth.