What is the recommended management of thyroid disease in a pregnant woman?

Management of Gestational Thyroid Disease

Hyperthyroidism Management

Use propylthiouracil (PTU) exclusively during the first trimester, then switch to methimazole for the second and third trimesters to minimize both congenital malformations and maternal hepatotoxicity. 1, 2

Medication Strategy

• PTU is preferred in the first trimester because methimazole carries higher risk of congenital malformations during organogenesis, while PTU has lower teratogenic potential despite its hepatotoxicity risk 1, 3, 4
• Switch to methimazole after the first trimester (weeks 13-14 onward) because continuing PTU increases maternal hepatotoxicity risk, and the critical period for fetal organ formation has passed 1, 2
• Maintain free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dose to avoid fetal hypothyroidism 1, 2, 3

Monitoring Protocol

• Check free T4 or FTI every 2-4 weeks to adjust medication dosage 1, 2, 3
• Once stable, check TSH every trimester 1
• Monitor for agranulocytosis (sore throat, fever) - if present, obtain complete blood count immediately and discontinue the thioamide 1, 3
• Monitor fetal heart rate and growth in women with Graves' disease 1

Adjunctive Treatment

• Beta-blockers (propranolol) can temporarily manage symptoms like tremors and palpitations until thioamide therapy reduces thyroid hormone levels 1, 3

Special Scenarios

Hyperemesis gravidarum with biochemical hyperthyroidism rarely requires treatment unless other clinical signs of hyperthyroidism are present 1, 3

Thyroid storm is a medical emergency requiring immediate treatment without waiting for laboratory confirmation:

• Administer PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care 5, 1, 3
• Avoid delivery during thyroid storm unless absolutely necessary 5, 3

Surgical Management

• Thyroidectomy is reserved only for women who do not respond to thioamide therapy or develop severe drug intolerance (agranulocytosis, severe hepatotoxicity) 1, 3
• If necessary, perform during the second trimester 5, 1, 3

Absolute Contraindications

• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation 1, 3, 4
• Women must wait four months after I-131 treatment before breastfeeding 5, 1, 3

Consequences of Inadequate Treatment

• Untreated or inadequately treated hyperthyroidism increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 1, 2, 3

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Hypothyroidism Management

Treat all pregnant women with elevated TSH using levothyroxine immediately to restore TSH to trimester-specific reference ranges, as untreated hypothyroidism increases risks of preeclampsia, low birth weight, and neuropsychological defects in offspring. 1, 2

Treatment Initiation

• Start levothyroxine immediately upon diagnosis to normalize TSH and prevent adverse maternal and fetal outcomes 2, 6
• Target TSH <2.5 mIU/L in the first trimester and maintain within pregnancy-specific reference ranges 7
• Never discontinue levothyroxine during pregnancy - hypothyroidism diagnosed during pregnancy should be promptly treated 6

Dose Adjustments

• Levothyroxine requirements often increase 25-50% during pregnancy, particularly in the first trimester 8
• Monitor serum TSH every 6-8 weeks during pregnancy and modify levothyroxine dose as needed 2
• Return to pre-pregnancy dose immediately after delivery as postpartum TSH levels are similar to preconception values 6

Risks of Undertreatment

• Maternal hypothyroidism increases risk of preeclampsia, spontaneous abortion, gestational hypertension, stillbirth, and premature delivery 1, 6
• Inadequate treatment is associated with low birth weight in neonates 1
• Maternal hypothyroidism from iodine deficiency increases risk of congenital cretinism (growth failure, mental retardation, neuropsychological defects) 1
• Undertreatment poses greater risks than overtreatment during pregnancy 2

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Postpartum Considerations

Neonatal Monitoring

• Inform the newborn's physician about maternal thyroid disease (both hyperthyroidism and hypothyroidism) due to risk of neonatal thyroid dysfunction 1, 2, 3

Breastfeeding

• Women treated with PTU or methimazole can safely breastfeed 1, 2, 4
• Levothyroxine is present in breast milk but no adverse effects on breastfed infants have been reported 6

Postpartum Thyroiditis

• Monitor for postpartum thyroiditis in women with history of thyroid dysfunction 1
• Postpartum thyroiditis is diagnosed by new onset of abnormal TSH level, abnormal free T4 level, or both 5
• Risk of permanent hypothyroidism is greatest in women with highest levels of TSH and antithyroid peroxidase antibodies 5

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Thyroid Nodules and Cancer in Pregnancy

• All thyroid nodules should be evaluated - up to 40% are found to be malignant 5
• Thyroidectomy can be performed, preferably during the second trimester 5
• Radiation therapy (I-131) should not be administered until after pregnancy 5

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Critical Pitfalls to Avoid

• Failing to switch from PTU to methimazole after first trimester increases risk of maternal hepatotoxicity 1, 2
• Using radioactive iodine during pregnancy causes fetal thyroid ablation 1, 3
• Failure to monitor thyroid function regularly during pregnancy may result in inadequate treatment 2
• Not increasing levothyroxine dose during pregnancy in women with pre-existing hypothyroidism leads to maternal and fetal complications 8