OBJECTIVE To evaluate the role of thyroid hormones in maintaining early pregnancy and to examine the association between thyroid physiological functions and immunological parameters. METHODS Forty-five pregnant women with a clinical diagnosis of threatened abortion and a live fetus and 30 normal pregnant women were included in the study. Blood samples were taken on admission to the hospital. The patients were divided retrospectively into two groups on the basis of outcome: 1) 31 women who did not miscarry (positive outcome) and 2) 14 women who miscarried (negative outcome). Plasma TSH, free triiodothyronine (fT3), free thyroxine (fT4), hCG, immunoglobulin (Ig) G and IgM concentrations and blood counts were determined in each patient. RESULTS Human chorionic gonadotropin was significantly higher in women who did not abort (39.4 +/- 16.9 IU/mL) than in women who miscarried (17.6 +/- 14.8 IU/mL, P < .001). Free thyroxine but not fT3 was lower in patients with negative outcome (1.25 +/- 0.26 ng/mL compared with 1.98 +/- 0.22 ng/mL, P < .001) and IgG and IgM plasma levels were higher (780 +/- 500 ng/mL compared with 470 +/- 300 ng/mL and 930 +/- 400 ng/mL compared with 650 +/- 280 ng/mL, respectively, P < .05). Plasma TSH levels were higher in patients with negative outcomes (1.72 +/- 0.84 mIU/mL compared to 1.01 +/- 0.41 mIU/mL, P < .001). Plasma concentrations of hCG and thyroid hormones were significantly correlated with peripheral blood lymphocyte and neutrophil counts only in the group of women who aborted. CONCLUSIONS Our results indicate that maternal immune response, trophoblast function, and maternal thyroid function are somehow correlated. The presence of low concentrations of hCG and fT4 and high levels of TSH and gamma globulins in women with threatened abortion suggests a negative outcome for the pregnancy.