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The hippocampus maintains a capacity for neurogenesis throughout life, a capacity that is reduced in models of adult onset hypothyroidism. The effects of developmental thyroid hormone (TH) insufficiency on neurogenesis in the adult hippocampus, however, has not been examined. Graded degrees of TH insufficiency were induced in pregnant rat dams by administration of 0, 3 or 10 ppm of 6-propylthiouracil (PTU) in drinking water from gestational day (GD) 6 until weaning. Body, brain, and hippocampal weight were reduced on postnatal day (PN) 14, 21, 78 and hippocampal volume was smaller at the 10 but not 3 ppm dose level. A second experiment examined adult hippocampal neurogenesis following developmental or adult onset hypothyroidism. Two male offspring from 0 and 3 ppm exposed dams were either" maintained on control water or exposed to 3 ppm PTU to create 4 distinct treatment conditions (Control-Control; Control-PTU, PTU-Control, PTU-PTU) based on developmental and adult exposures. Beginning on the 28th day of adult exposure to 0 or 3 ppm PTU, bromodeoxyuridine (BrdU, 50 mg/kg, ip) was administered twice daily for 5 days, and one male from each treatment was sacrificed 24 h and 28 days after the last BrdU dose and brains processed for immunohistochemistry. Although no volume changes were seen in the hippocampus of the neonate at 3 ppm, thinning of the granule cell layer emerged in adulthood. Developmental TH insufficiency produced a reduction in newly born cells, reducing BrdU+ ve cells at 1 with no further reduction at 28-days post-BrdU. Similar findings were obtained using the proliferative cell marker Ki67. Neuronal differentiations was also altered with fewer doublecortin (Dcx) expressing cells and a higher proportion of immature Dcx phenotypes seen after developmental but not adult TH insufficiency. An impaired capacity for neurogenesis may contribute to impairments in synaptic plasticity and cognitive deficits previously reported by our laboratory and others following moderate degrees of developmental TH insufficiency induced by this PTU model. Published by Elsevier B.V.