n engl j med
new england journal
Timing and Magnitude of Increases
in Levothyroxine Requirements
during Pregnancy in Women
Erik K. Alexander, M.D., Ellen Marqusee, M.D., Jennifer Lawrence, M.D.,
Petr Jarolim, M.D., Ph.D., George A. Fischer, Ph.D., and P. Reed Larsen, M.D.
From the Thyroid Section, Division of Endo-
crinology, Diabetes, and Hypertension, De-
partment of Medicine (E.K.A., E.M., P.R.L.),
and the Division of Clinical Laboratories,
Department of Pathology (P.J., G.A.F.),
Brigham and Women’s Hospital and Har-
vard Medical School, Boston; and Valdos-
ta Specialty Clinic, Valdosta, Ga. (J.L.). Ad-
dress reprint requests to Dr. Alexander at
the Thyroid Section, Division of Endocrinol-
ogy, Diabetes, and Hypertension, Depart-
ment of Medicine, Brigham and Women’s
Hospital and Harvard Medical School, 221
Longwood Ave., 2nd Fl., Boston, MA 02115,
or at firstname.lastname@example.org.
N Engl J Med 2004;351:241-9.
Copyright © 2004 Massachusetts Medical Society.
Hypothyroidism during pregnancy has been associated with impaired cognitive devel-
opment and increased fetal mortality. During pregnancy, maternal thyroid hormone re-
quirements increase. Although it is known that women with hypothyroidism should
increase their levothyroxine dose during pregnancy, biochemical hypothyroidism occurs
in many. In this prospective study we attempted to identify precisely the timing and
amount of levothyroxine adjustment required during pregnancy.
Women with hypothyroidism who were planning pregnancy were observed prospective-
ly before and throughout their pregnancies. Thyroid function, human chorionic gonad-
otropin, and estradiol were measured before conception, approximately every two weeks
during the first trimester, and monthly thereafter. The dose of levothyroxine was in-
creased to maintain the thyrotropin concentration at preconception values throughout