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Previous Volume 351:241-249 July 15, 2004 Number 3 Next

	Full Text PDF PDA Full Text PowerPoint Slide Set Editorial by Toft, A. Add to Personal Archive Add to Citation Manager Notify a Friend E-mail When Cited E-mail When Letters Appear PubMed Citation

ABSTRACT

Background Hypothyroidism during pregnancy has been associated with impaired cognitive development and increased fetal mortality. During pregnancy, maternal thyroid hormone requirements 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.

Methods Women with hypothyroidism who were planning pregnancy were observed prospectively before and throughout their pregnancies. Thyroid function, human chorionic gonadotropin, and estradiol were measured before conception, approximately every two weeks during the first trimester, and monthly thereafter. The dose of levothyroxine was increased to maintain the thyrotropin concentration at preconception values throughout pregnancy.

Results Twenty pregnancies occurred in 19 women and resulted in 17 full-term births. An increase in the levothyroxine dose was necessary during 17 pregnancies. The mean levothyroxine requirement increased 47 percent during the first half of pregnancy (median onset of increase, eight weeks of gestation) and plateaued by week 16. This increased dose was required until delivery.

Conclusions Levothyroxine requirements increase as early as the fifth week of gestation. Given the importance of maternal euthyroidism for normal fetal cognitive development, we propose that women with hypothyroidism increase their levothyroxine dose by approximately 30 percent as soon as pregnancy is confirmed. Thereafter, serum thyrotropin levels should be monitored and the levothyroxine dose adjusted accordingly.

Source Information

From the Thyroid Section, Division of Endocrinology, Diabetes, and Hypertension, Department 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 Harvard Medical School, Boston; and Valdosta Specialty Clinic, Valdosta, Ga. (J.L.).

Address reprint requests to Dr. Alexander at the Thyroid Section, Division of Endocrinology, Diabetes, and Hypertension, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, 221 Longwood Ave., 2nd Fl., Boston, MA 02115, or at ekalexander{at}partners.org.

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