Thyroid hormones (THs) are essential for brain development, but few rodent models exist that link TH inefficiency to apical neurodevelopmental endpoints. We have previously described a structural anomaly, a heterotopia, in the brains of rats treated in utero with propylthiouracil (PTU). However, how the timing of an exposure relates to this birth defect is unknown. This study seeks to understand how various temporal treatments of the mother relates to TH insufficiency and adverse neurodevelopment of the offspring. Pregnant rats were exposed to PTU (0 or 3 ppm) through the drinking water from gestational day 6 until postnatal day (PN) 14. On PN2 a subset of pups was cross-fostered to a dam of the opposite treatment, to create 4 conditions: pups exposed to PTU prenatally, postnatally, during both periods, or not at all (control). Both PTU and TH concentrations were characterized in the mother and offspring over time, to capture the dynamics of a developmental xenobiotic exposure. Brains of offspring were examined for heterotopia presence and severity, and adult littermates were assessed for memory impairments. Heterotopia were observed under conditions of prenatal exposure, and its severity increased in animals in the most prolonged exposure group. This malformation was also permanent, but not sex biased. In contrast, behavioral impairments were limited to males, and only in animals exposed to PTU during both the gestational and postnatal periods. This suggests a distinct TH-dependent etiology for both phenotypes, and illustrates how timing of hypothyroxinemia can induce abnormal brain structure and function.