The lymphocyte subsets and the percentages of activated T cells or regulatory T cells were observed during the course of postpartum thyroiditis (PPT). Heparin anticoagulant vein bloods were collected at 3, 6, 9 and 12 months postpartum consequently from 27 PPT patients and 23 normal postpartum subjects. Lymphocyte surface antigen CD3, CD4, CD8, HLA-DR and CD25 were stained in appropriate combination and detected with fluorescence-activated cell sorter analysis. The percentage of CD4 was significantly lower in both PPT groups with biphasic diseases and isolated hypothyroidism at 3 months postpartum as compared to control postpartum women separately (both P < 0.05). Then, decreased CD4/CD8 ratios were also appeared in these groups. Patients with both positive TPOAb and TgAb had higher percentage of activated T (HLA-DR(+)CD3(+)) cells compared to control postpartum women at 3 months postpartum (P < 0.05). The percentage of activated T cells correlated with a raised percentage of CD8(+) T cell subset (P < 0.001) and a decreased percentage of regulatory T (CD25(+)CD4(+)) cells (P < 0.01). The percentages of regulatory T cells were significantly higher in control postpartum women at 3, 6 and 9 months postpartum compared with non-postpartum women (P < 0.05). However, it was lower in PPT patients at 3 months compared to itself at 6 and 9 months postpartum (P < 0.05). In the early postpartum period of PPT patients, a reduced helper/inducer T cell subset, an increased percentage of activated T cells and a reduced percentage of regulatory T cells were reported, indicating that T cells may play a key role in the pathogenesis of PPT.