The effect of abortion on serum concentration of thyroid hormones was studied in 11 normal women. After abortion, serum free and total T3 fall significantly, while free and total reverse T3, T4, TSH and TBG in serum show no significant variation. These data suggest that surgical stress in normal subjects may only influence 5'-deiodinase activity, and that the peripheral conversion of T4 to active T3 and inactive rT3 are controlled by different mechanisms.