Partial restoration of cerebral myelination of the congenitally hypothyroid mouse by parenteral or breast milk administration of thyroxine.

We attempted to define whether thyroid hormone can ameliorate the cerebral hypomyelination present in the congenitally hypothyroid (hyt) neonatal mouse, and to define the critical time period during early postnatal life when thyroxine (T4) is essential for myelin formation. We administered T4 to the hyt mouse by breast milk during the first 20 days of postnatal life, and through the diet during the second 20 days of postnatal life. Positive results were obtained only when hormone was given during the first 20 days of postnatal life. A distinct increase in cerebral 2',3'-cyclic nucleotide 3'-phosphohydrolase activity was noted, and brain sections stained for myelin basic protein correlated with the biochemical findings. The later administration of hormone through diet was ineffective.