Mild Gestational Hypothyroidism in Mice Has Transient Developmental Effects and Long-Term Consequences on Neuroendocrine Systems.

Thyroid hormones (TH) play a key role in fetal brain development. While severe thyroid dysfunction, has been shown to cause neurodevelopmental and reproductive disorders, the rising levels of TH-disruptors in the environment in the past few decades have increased the need to assess effects of subclinical (mild) TH insufficiency during gestation. Since embryos do not produce their own TH before mid-gestation, early development processes rely on maternal production. Notably, the reproductive network governed by gonadotropin-releasing hormone (GnRH) neurons develops during this critical period. The risk of mild maternal hypothyroidism on the development of GnRH neurons and long-term effect on neuroendocrine function in the offspring was investigated using a mouse model of gestational hypothyroidism induced by methimazole (MMI) treatment. MMI treatment during gestation led to reduced litter size, consistent with increased miscarriages due to hypothyroidism. E12/13 embryos, collected from MMI-treated dams, had a decreased number of GnRH neurons, but the migration of the remaining GnRH neurons was normal. Cell proliferation was reduced in the vomeronasal organ (VNO), correlating with the reduced number of GnRH neurons detected in this region. Using a GnRH cell line confirmed attenuated proliferation in the absence of T3. Pups born from hypothyroid mothers had normal postweaning growth and estrus cycles, yet adult offspring had significantly more cells expressing estrogen receptor alpha in the arcuate nucleus. Notably, by adulthood, GnRH cell number and distribution was comparable with nontreated controls indicating that compensatory mechanisms occurred after E13. Overall, our work shows that mild TH disruption during gestation transiently affects proliferation of the pool of GnRH neurons within the VNO and has a long-term impact on neuroendocrine systems.