A volumetric analysis of the brain and hippocampus of rats rendered perinatal hypothyroid

The thyroid hormone is essential for the proper development of the central nervous system (CNS). Hormone deficiency during CNS development causes neurological abnormalities in the brain. The hippocampus is one of the brain regions vulnerable to hormone deficiency, and the volume of dentate gyrus (DG) and cornu ammonis (CA) are reduced by transient hypothyroidism during CNS development. However, it remains unclear whether transient hypothyroidism specifically reduces the whole hippocampal volume. In the present study, we used magnetic resonance imaging (MRI) to examine the effects of perinatal hypothyroidism on the ratio of hippocampal volume to brain volume as well as brain and hippocampal volumes overall. Perinatal hypothyroidism was induced by adding the anti-thyroid drug, methimazole, to the drinking water of pregnant dams from gestational day 15 to postnatal day 21. The MRI experiment was conducted when the rats were between 7 and 11 months old. The results showed reductions of the hippocampal and brain volume of the treated group. However, the ratio of hippocampal volume to brain volume was comparable between the control and treated groups. These results indicate that perinatal hypothyroidism minimizes the brain as a whole, but does not minimize the hippocampus in particular.