The thyroid is an important and dynamic gland during pregnancy, and the current guidelines from the American Thyroid Association (ATA) provide a review of changes in the thyroid during pregnancy. The gland can increase 10% in size during pregnancy in countries with adequate sources of iodine, and even more so in iodine-poor countries.
This increase in size is accompanied by a 50% increase in the production of thyroid hormones and a concomitant 50% increase in the iodine requirement. In addition, 10% of pregnant women in the first trimester will test serologically positive for thyroid peroxidase (TPO) or thyroglobulin antibody, and 16% of these women may go on to have hypothyroidism during pregnancy.
In addition, postpartum thyroiditis develops in another 33% to 50% of women with thyroid-related antibodies.
These changes complicate the assessment and management of potential thyroid illness during pregnancy. The current guidelines suggest best practices to help clinicians.
Study Synopsis and PerspectiveHypothyroidism during pregnancy is harmful to maternal and fetal health and to the child's future intellectual development, according to new guidelines of the ATA reported online July 25 in Thyroid.
The new recommendations address the diagnosis and management of thyroid disease during pregnancy and the postpartum period.
"Pregnancy has a profound impact on the thyroid gland and thyroid function," said Alex Stagnaro-Green, MD, MHPE, from the Department of Medicine and Obstetrics/Gynecology, George Washington University School of Medicine and Health Sciences in Washington, DC, and chair of the ATA Taskforce on Thyroid Disease During Pregnancy and Postpartum, in a news release. "In essence, pregnancy is a stress test for the thyroid, resulting in hypothyroidism in women with limited thyroidal reserve or iodine deficiency."
During pregnancy, the thyroid gland may enlarge by 10% in countries where iodine sources are sufficient, and to a greater extent in iodine-poor countries. Production of thyroid hormones and iodine requirement each increase by approximately 50% during pregnancy. Evidence reviewed by the Taskforce included findings from clinical trials showing the harmful effects of subclinical thyroid disease, as well as overt hypothyroidism and hyperthyroidism, on pregnancy and on maternal and fetal health.
Data from ongoing studies are elucidating the association between miscarriage and preterm delivery in women with normal thyroid function who test positive for thyroid peroxidase (TPO) antibodies. During the first trimester, approximately 1 in 10 pregnant women develops antibodies to TPO or to thyroglobulin, and hypothyroidism develops in roughly 16% of these women.
The long-term effects of postpartum thyroiditis, which occurs in approximately 33% to 50% of women with thyroid-related antibodies, are also reviewed.
"These important guidelines were developed by a panel of international experts representing the disciplines of endocrinology, obstetrics and gynecology, and nurse midwives," said ATA president Gregory A. Brent, MD, professor of medicine and physiology at the David Geffen School of Medicine, University of California Los Angeles. "This broad representation of providers that care for pregnant women will significantly increase the impact of these guidelines and translation of findings from the most recent research to clinical practice."
The new guidelines include 76 specific recommendations highlighting the role of thyroid function tests, hypothyroidism, thyrotoxicosis, iodine, thyroid antibodies and miscarriage/preterm delivery, thyroid nodules and cancer, postpartum thyroiditis, recommendations on screening for thyroid disease during pregnancy, and areas for future research.
Among the specific recommendations are the following:
- Oral levothyroxine is indicated for women with overt hypothyroidism, which is associated with greater risks for fetal loss and premature birth, and for those with subclinical hypothyroidism who test positive for TPO antibodies.
- To treat maternal hypothyroidism, use of triiodothyronine, desiccated thyroid, or other thyroid preparations is strongly not recommended.
- Women who are already receiving thyroid replacement therapy should increase their dose by 25% to 30% when they become pregnant.
- Women with subclinical hypothyroidism in pregnancy who are not initially treated should be monitored for progression to overt hypothyroidism. Serum thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels should be measured approximately every 4 weeks until 16 to 20 weeks' gestation and at least once between 26 and 32 weeks' gestation.
- In the first trimester, normal range for TSH level is 0.1 to 2.5 mIU/L; this level increases to 0.2 to 3.0 mIU/L in the second trimester and 0.3 to 3.0 mIU/L in the third trimester.
- Serum levels of FT4 during pregnancy should be measured with online solid phase extraction–liquid chromatography, or tandem mass spectrometry on serum dialysate or ultrafiltrate.
- Treatment is not needed for women with isolated low FT4 levels.
- During pregnancy and lactation, the minimal suggested daily recommended allowance for iodine is 250 μg. The risk for fetal hypothyroidism may increase when total daily iodine intake from diet and/or supplements is or exceeds 500 μg.
- Pregnant women should not undergo radioactive iodine thyroid scanning, but fine-needle aspiration of thyroid nodules may be performed if indicated.
- Evidence is insufficient to recommend for or against routine screening for antithyroid antibodies among women with miscarriage, or universal TSH screening in the first trimester. However, screening for FT4 level is not recommended.
- Antithyroid drugs are not indicated for gestational hyperthyroidism, which can be managed supportively. However, women in the first trimester in whom Graves' hyperthyroidism develops should receive propylthiouracil.
- During the thyrotoxic phase of postpartum thyroiditis, antithyroid drugs are not needed. To monitor women for development of hypothyroidism once the thyrotoxic phase has ended, screening should be performed every 2 months for 1 year.
- At approximately 6 to 12 months after starting treatment of postpartum thyroiditis for their patients, clinicians should try to taper thyroid replacement therapy.
Thyroid. Published online July 25, 2011.