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Adequate iodine intake during pregnancy is critical for maintaining maternal and fetal thyroid function and development. There are only limited data from iodine-balance studies to inform iodine requirements during pregnancy.

Adequate maternal thyroid function is important for an uncomplicated pregnancy. Although multiple observational studies have evaluated the association between thyroid dysfunction and hypertensive disorders of pregnancy, the methods and definitions of abnormalities in thyroid function tests were heterogeneous, and the results were conflicting. We aimed to examine the association between abnormalities in thyroid function tests and risk of gestational hypertension and pre-eclampsia.

Thyroid disorders are prevalent in pregnant women. Furthermore, thyroid hormone has a critical role in fetal development and thyroid dysfunction can adversely affect obstetric outcomes. Thus, the appropriate management of hyperthyroidism, most commonly caused by Graves disease, and hypothyroidism, which in iodine sufficient regions is most commonly caused by Hashimoto thyroiditis, in pregnancy is important for the health of both pregnant women and their offspring. Gestational transient thyrotoxicosis can also occur during pregnancy and should be differentiated from Graves disease. Effects of thyroid autoimmunity and subclinical hypothyroidism in pregnancy remain controversial. Iodine deficiency is the leading cause of hypothyroidism worldwide. Despite global efforts to eradicate iodine deficiency disorders, pregnant women remain at risk of iodine deficiency due to increased iodine requirements during gestation. The incidence of thyroid cancer is increasing worldwide, including in young adults. As such, the diagnosis of thyroid nodules or thyroid cancer during pregnancy is becoming more frequent. The evaluation and management of thyroid nodules and thyroid cancer in pregnancy pose a particular challenge. Postpartum thyroiditis can occur up to 1 year after delivery and must be differentiated from other forms of thyroid dysfunction, as treatment differs. This Review provides current evidence and recommendations for the evaluation and management of thyroid disorders in pregnancy and in the postpartum period.

Both hyperthyroidism and hypothyroidism can have adverse effects in pregnancy. The most common causes of thyrotoxicosis in pregnancy are gestational transient thyrotoxicosis and Graves’ disease. It is important to distinguish between these entities as treatment options differ. Women of reproductive age who are diagnosed with Graves’ disease should be counseled regarding the impact of treatment options on a potential pregnancy. Although the absolute risk is small, antithyroid medications can have teratogenic effects. Propylthiouracil appears to have less severe teratogenicity compared to methimazole and is therefore favored during the first trimester if a medication is needed. Women should be advised to delay pregnancy for at least 6 months following radioactive iodine to minimize potential adverse effects from radiation and ensure normal thyroid hormone levels prior to conception. As thyroid hormone is critical for normal fetal development, hypothyroidism is associated with adverse obstetric and child neurodevelopmental outcomes. Women with overt hypothyroidism should be treated with levothyroxine (LT4) to a thyrotropin (thyroid-stimulating hormone; TSH) goal of <2.5 mIU/L. There is mounting evidence for associations of maternal hypothyroxinemia and subclinical hypothyroidism with pregnancy loss, preterm labor, and lower scores on child cognitive assessment. Although there is minimal risk of LT4 treatment to keep TSH within the pregnancy-specific reference range, treatment of mild maternal thyroid hypofunction remains controversial, given the lack of clinical trials showing improved outcomes with LT4 treatment.

Adequate transplacental passage of maternal thyroid hormone is important for normal fetal growth and development. Maternal overt hypothyroidism and hyperthyroidism are associated with low birthweight, but important knowledge gaps remain regarding the effect of subclinical thyroid function test abnormalities on birthweight—both in general and during the late second and third trimester of pregnancy. The aim of this study was to examine associations of maternal thyroid function with birthweight.

Iodine is essential for the synthesis of thyroid hormones, which regulate cell metabolism, growth, and development. Although iodine deficiency (ID) causes adverse health effects across the lifespan, it is particularly problematic in pregnancy, when it can lead to irreversible fetal brain damage. A high prevalence of severe ID, manifesting as endemic goiter and cretinism, predated the arrival of European explorers in the Americas. Early 20th century surveys showed that most countries in the Western Hemisphere had regions with a goiter prevalence >50%. In North America, the introduction of iodized salt led to the elimination of ID by the 1950s. Although most Latin American countries passed laws mandating salt iodization in the 1950s–1960s, initial programs met with limited success because laws were unenforced, monitoring was absent, and the importance of iodine nutrition was inadequately communicated. A renewed interest in ID prevention arose in the 1970s–1980s, when 3 Andean countries were the first in Latin America to implement effective salt iodization programs. Over the last 3 decades there has been a stronger political commitment to ID prevention across the region, alignment with the broader nutrition and development agenda, and a widespread recognition of optimal iodine nutrition as a fundamental human right. Currently, 92% of households in Latin America consume adequately iodized salt, and urinary iodine concentrations in schoolchildren reflect optimal iodine nutrition across the region. However, additional work remains. It is essential to ensure ongoing government commitment; to monitor population iodine status and the production, quality, and household consumption of iodized salt; and to maintain advocacy and communication strategies. Universal salt iodization programs must be harmonized with efforts to reduce salt intake for cardiovascular disease prevention. Ensuring optimal iodine nutrition in pregnant women, who may remain deficient even when intakes in schoolchildren are optimized, requires particular attention.

(Abstracted from JAMA2019;322(7):632–641)Preterm birth complicates 5% to 15% of births worldwide, and although it is the most important direct cause of morbidity and mortality in children younger than 5 years, no known risk factors can be identified in the majority of cases. However, overt hypothyroidism and hyperthyroidism during pregnancy are well-known risk factors for preterm birth, and as such, the purpose of this study was to assess whether thyroid function test abnormalities and thyroid peroxidase (TPO) antibody positivity were associated with preterm birth.

Understanding of changes in thyroid function and the consequences of thyroid disease during pregnancy has rapidly grown in the past two decades, and revised American Thyroid Association guidelines on this topic were published in 2017. This Review explores the association between thyroid autoimmunity and complications during and after pregnancy. Thyroid autoimmunity refers to the presence of antibodies to thyroperoxidase or thyroglobulin, or thyroid-stimulating hormone receptor antibodies (TRAbs), or a combination of these, and is present in up to 18% of pregnant women. Thyroid antibodies in pregnant women with normal functioning thyroids (ie, euthyroid) have been associated with several complications, including miscarriage and premature delivery. Treatments to improve pregnancy outcomes are being studied. Whether thyroid antibodies are associated with infertility and assisted reproductive technology outcomes is unclear; although, treatment with low doses of levothyroxine, which is usually used to treat hypothyroidism, can be considered in such situations. Additionally, thyroid antibodies have been associated with other neonatal and maternal complications. All these associations require confirmation in larger prospective studies, and their pathogenic mechanisms need to be better understood. Post-partum thyroiditis is substantially more frequent in women who have thyroid antibodies during pregnancy than in those who do not have thyroid antibodies; however, whether treatment can prevent post-partum thyroiditis in women who are or have been antibody positive is unknown. Finally, TRAbs cross the placenta from the mother to the fetus and can cause fetal or neonatal hyperthyroidism. Therefore, women who are positive for TRAbs during pregnancy should be monitored.

Iodine deficiency is the leading cause of preventable neurodevelopmental delay in children worldwide and a possible public health concern in Haiti.

An awareness of the gestational changes to thyroid physiology and the impact of uncontrolled thyroid disease on pregnancy and infant outcome is essential for the successful management of hypothyroidism and hyperthyroidism. This review summarizes strategies for the management of thyroid disease in pregnancy and post partum, and it highlights areas where there is still a lack of consensus.