Thyroid function and ultrasonographically determined thyroid volume were investigated at regular intervals during pregnancy and the first postpartum year in 20 women without thyroid autoantibodies. Serum total thyroxine and total triiodothyronine levels were increased and free thyroxine index and free triiodothyronine index levels were decreased, whereas serum thyrotropin level was unaltered during pregnancy when compared with postpartum levels. Thyroid volume was increased during pregnancy with a mean variation of 30% between maximum values (24.1 +/- 2.2 ml, mean +/- SEM, thirty-sixth week of pregnancy) and minimum values (18.4 +/- 2.0 ml, 12 months post partum) (p less than 0.01). In conclusion, our study demonstrates a goitrogenic effect of pregnancy unexplained by alterations in thyroid function variables.