The interpretation of the changes in thyroid hormone concentrations during normal pregnancy is a matter of debate involving, in some geographical regions, enhanced thyroid activity in early pregnancy and a hypothyroid state in the third trimester. A clinically detectable increase in thyroid size has been found in areas of mild iodine deficiency ('goitre of pregnancy'), but not in iodine replete areas. A prospective study. We have studied thyroid size and function in normal pregnant women living in an iodine replete area. Healthy women before and during a normal pregnancy resulting from artificial insemination (n = 10) and other women during the normal menstrual cycle (n = 11), in the iodine replete area of Amsterdam. Thyroid volume was measured by ultrasonography. Plasma T4, free T3, free reverse T3, TSH, thyroxine binding globulin, hCG, progesterone and thyroid autoantibodies were measured. Thyroid volume did not change during pregnancy (data given before pregnancy and during 1st, 2nd and 3rd trimesters, respectively: 10.3 +/- 5.1, 10.6 +/- 4.4, 9.6 +/- 3.8 and 9.4 +/- 3.0 ml, NS). Free T4 and free T3 levels declined during pregnancy (13.7 +/- 2.0, 13.5 +/- 4.1, 11.2 +/- 2.8, 10.2 +/- 1.6 pmol/l, P = 0.005; 4.55 +/- 0.63, 4.64 +/- 0.88, 3.72 +/- 0.67 and 4.01 +/- 0.75 pmol/l, P = 0.003), whereas free reverse T3 levels increased during pregnancy (0.16 +/- 0.04, 0.19 +/- 0.07, 0.14 +/- 0.03 and 0.20 +/- 0.07 pmol/l, P = 0.001). Thyroglobulin levels remained unchanged. Thyroid hormones and thyroid volume did not differ between follicular and luteal phases of the menstrual cycle. Thyroid volume does not increase during pregnancy in iodine-replete areas. The decrease in free T4 and free T3 and the increase in free reverse T3 concentrations during pregnancy resemble the changes in thyroid hormones seen in non-thyroidal illness. This could be a physiological adaptation enabling energy conservation during the high metabolic demands of pregnancy.