What are the current guidelines for screening, diagnosing, and managing overt and subclinical hypothyroidism and overt hyperthyroidism in pregnant women, including recommended timing of thyroid function tests, treatment thresholds, medication choices and dosing, monitoring intervals, and postpartum follow‑up?

Management of Thyroid Disorders in Pregnancy

For pregnant women with hypothyroidism, immediately increase levothyroxine by approximately 30% by 4-6 weeks' gestation and monitor closely, while for hyperthyroidism, use propylthiouracil in the first trimester then switch to methimazole in the second and third trimesters to minimize teratogenic risk. 1

Screening and Diagnosis

Who to Screen

• Routine universal screening is not recommended, but women with risk factors and symptoms of thyroid disease should be screened 1
• Screen all women with pre-existing thyroid disease, thyroid antibodies, or symptoms suggestive of thyroid dysfunction 2, 3

Diagnostic Considerations

• Use trimester-specific reference ranges for TSH and free T4, as non-pregnant reference intervals lead to erroneous assessment 4
• First and second trimester reference values differ significantly from non-pregnant values due to physiological changes 2
• For central hypothyroidism, free T4 levels become the primary monitoring parameter rather than TSH 5

Hypothyroidism Management

Pre-Conception Optimization

• Women must be adequately treated before attempting conception to avoid first-trimester complications, as hypothyroidism in the first trimester is associated with cognitive impairment in children 1, 6, 5
• Many patients with hypothyroidism are inadequately treated before pregnancy, which poses significant risks 1, 5

Treatment Protocol During Pregnancy

• Increase levothyroxine dosage by 30% or more by 4-6 weeks' gestation 1, 5
• Use a specific brand of levothyroxine rather than switching between generic formulations 4
• Monitor thyroid function frequently during the first trimester, as many women demonstrate increased thyroid hormone requirements early in pregnancy 4

Monitoring Intervals

• Check thyroid function every 4 weeks until TSH levels stabilize 6
• Continue frequent monitoring throughout pregnancy, as dosage requirements often change 4
• For central hypothyroidism specifically, monitor free T4 rather than relying on TSH 5

Risks of Inadequate Treatment

• Untreated or inadequately treated hypothyroidism increases risk of:
  • Preterm birth 1, 6
  • Low birth weight 1, 6, 5
  • Placental abruption 1, 6, 5
  • Fetal death 1, 6, 5
  • Preeclampsia 5
  • Congenital cretinism with severe maternal hypothyroidism, resulting in growth failure, mental retardation, and neuropsychologic defects 5

Treatment Benefits

• Women who are adequately treated before pregnancy and those diagnosed and treated early in pregnancy have no increased risk of perinatal morbidity 1, 6, 5

Hyperthyroidism Management

Pre-Conception Goals

• Achieve euthyroidism before pregnancy, as hyperthyroidism can result in significant maternal and neonatal morbidity 1
• Outcomes correlate directly with disease control 1

Medication Selection by Trimester

• First trimester: Use propylthiouracil to avoid methimazole-associated teratogenicity 1
• Second and third trimesters: Switch to methimazole to avoid propylthiouracil-associated hepatotoxicity 1
• Antithyroid drugs are the main therapeutic form administered in pregnant women 7

Treatment Goals

• Aim to achieve a subclinical hyperthyroidism state rather than complete normalization 7
• Overt hyperthyroidism requires treatment, though mild hyperthyroidism may be well tolerated 4
• Close monitoring is required after initiating thionamides to avoid maternal and fetal hypothyroidism 4

Alternative Treatment Options

• Radioactive iodine therapy is absolutely contraindicated during pregnancy 4, 7
• Thyroidectomy should be limited to severe non-responsive thyroid dysfunction and is generally safe in the second trimester in an appropriately prepared woman 4, 7
• Limited data exists about the role and safety of oral contrast agents, iodine, amiodarone, and perchlorate 4

Neonatal Considerations

• Newborns of mothers with Graves' disease require monitoring for neonatal thyroid dysfunction due to transplacental passage of maternal thyroid-stimulating antibodies 6, 8
• Offspring may rarely develop fetal/neonatal hyperthyroidism, requiring close collaboration between endocrinologists, obstetricians, and neonatologists 8

Thyroid Autoimmunity Without Overt Dysfunction

• Approximately 5-18% of pregnant women exhibit elevated thyroid-specific antibodies 2
• These women may have restricted thyroid reserve, followed by hypothyroxinemia and/or TSH increase during pregnancy 2
• Maternal thyroid dysfunction or only the presence of thyroid-specific antibodies is associated with increased risk for early abortion, preterm delivery, and neonatal morbidity 2
• The incidence of miscarriage, preterm delivery, small for date offspring, and delayed neuropsychological development may be increased 2

Postpartum Follow-Up

• Infants born to mothers with thyroid disorders should have thyroid function assessed, as serial monitoring and treatment may be necessary 3
• Breastfeeding is considered safe in mothers taking antithyroid medications 3

Critical Pitfalls to Avoid

• Do not use non-pregnant reference ranges for thyroid function tests during pregnancy 4
• Do not rely on TSH alone for monitoring central hypothyroidism—free T4 is the primary parameter 6, 5
• Do not wait to increase levothyroxine—adjustments should begin by 4-6 weeks' gestation 5
• Do not assume pre-pregnancy dosing is adequate—most women require a 30% or more increase 5
• Do not switch between levothyroxine brands or generics during pregnancy 4
• Pregnant women under levothyroxine treatment are often undertreated or overtreated without proper monitoring 2