To evaluate a possible role of thyroid hormones in maintaining early pregnancy, serum levels of thyroid hormones, TSH and thyroxine-binding globulin (TBG) in 32 patients with a clinical diagnosis of threatened abortion were compared between two groups of patients with favorable and unfavorable pregnancy outcome. Serum levels of T4, T3, free T4 and free T3 levels determined at the onset of clinical signs of threatened abortion were found to be significantly lower in patients (N = 11) who subsequently aborted compared to patients (N = 21) who did not. Serum TSH levels did not differ between the two groups. Serum TBG levels in the patients who subsequently aborted were lower compared to patients with favorable pregnancy outcome. Furthermore, serum levels of T4 and T3 at the onset of threatened abortion in patients who subsequently did not abort were significantly higher compared to levels before pregnancy, whereas little increase in serum T4 and T3 levels relative to the prepregnant levels was observed at the onset of clinical signs in the patients who subsequently aborted. These data imply a possible role of thyroid hormones in maintaining early pregnancy, and suggest that maternal serum level of thyroid hormone may be one of the endocrine factors responsible for the outcome of threatened abortion.