Hypothyroidism prolongs mitotic activity in the post-natal mouse brain.

Circulating T(4) and T(3) were measured during the first three post-natal weeks in the mouse and found to increase in a triphasic manner. The first increase occurred at post-natal day 6 and was simultaneous with a decrease in bromodeoxyuridine incorporation in areas showing post-natal mitosis. We investigated whether there was a causal relationship between increased thyroid hormone levels and decreased proliferation by inducing hypothyroidism in dams and progeny. Hypothyroidism prolonged mitotic activity in the olfactory bulb, hippocampus, subventricular zone and the cerebellar cortex. This suggests that the increase in T(3) at the end of the first postnatal week is implicated in terminating progenitor proliferation in many parts of the mouse brain.