Thyroid hormones (THs) regulate adult hippocampal neurogenesis, a process that involves both cell populations that dynamically switch between pools of proliferative and quiescent cells, and cells that definitely leave the cell cycle to maturate into granular neurons. This investigation was carried out to determine the role of THs on the mitotic activity of specific proliferative cell populations and the preservation of non-proliferative cells participating in the neurogenic process of the dentate gyrus (DG) of the hippocampus. Hypothyroidism was induced in male adult Wistar rats with methimazole for 28 days. We quantified the total number of proliferative cells (BrdU+), proliferative type 1 (BrdU+/GFAP+), and 2b and 3 (BrdU+/DCX+) cells. Early non-proliferative cells (BrdU-/DCX+ cells lacking dendritic process), postmitotic neuroblasts (Tuj 1+ cells lacking dendritic process), and immature granular neurons (IGN; DCX+ or Tuj 1+ and the presence of dendritic processes into granular or molecular layer) were also included. The evidence showed that the proliferation of Type 1, 2b and 3 cells is not modified by hypothyroidism. In contrast, hypothyroidism reduced the number of early non-proliferative cells and also leads to a decrement in the number of IGN. Our results show that proliferative cells of the DG are not sensitive to thyroid perturbations. However, THs are essential to preserve cell populations that leave the cell cycle in the DG of the hippocampus.