Understanding of how maternal thyroid inadequacy during early gestation poses a risk for developmental outcomes is still a challenge for the neuro-endocrine community. Early neocortical neurogenesis is accompanied by maternal thyroid hormone transfer to fetal brain, appearance of thyroid hormone receptors and absence of anti neurogenesis signals, followed by optimization of neuronal numbers through apoptosis. However, the effects of thyroid hormone (TH) deprivation on neurogenesis and neuronal cell death before the onset of fetal thyroid are still not clear. We show that maternal TH deficiency during early gestational period causes massive premature elevation in the expression of neuronal nitric oxide synthase (nNOS) with an associated neuronal death in embryonic rat neocortex. Maternal hypothyroidism was induced by feeding methimazole (0.025% w/v) in the drinking water to pregnant Sprague Dawley rats from embryonic day (ED) 6. Cerebral cortices from fetuses were harvested at different embryonic stages (ED14, ED16 and ED18) of hypothyroid and euthyroid group. Immunoblotting and Real-Time PCR results showed that both protein and RNA levels of nNOS were pre-maturely increased under maternal hypothyroidism and showed reversibility upon thyroxine administration. Immunohistochemistry revealed an increased nNOS immuno-reactivity in both the cortical plate and proliferative zone of neo-cortex along with a corroborative decrease in the Microtubule associated protein-2 (MAP-2) positive neurons under maternal TH insufficiency. Results combined, put forth nNOS as a novel target of maternal thyroid hormone action in embryonic neocortex and underscore the importance of prenatal screening and timely rectification of maternal thyroid hormone insufficiency even of a moderate degree.