What is the management of pregnancy-associated hyperthyroidism?

Management of Pregnancy-Associated Hyperthyroidism

Pregnancy-associated hyperthyroidism should be treated with thioamide medications, using propylthiouracil (PTU) in the first trimester followed by a switch to methimazole for the second and third trimesters, with the goal of maintaining free T4 or FTI in the high-normal range using the lowest possible effective dosage. 1

Diagnosis and Initial Assessment

• The initial diagnostic workup should include TSH and FT4 or FTI testing for pregnant women with suspected hyperthyroidism 2
• Rule out gestational transient thyrotoxicosis (associated with hyperemesis gravidarum), which can present with biochemical hyperthyroidism but rarely requires treatment 1
• Graves' disease is the most common cause of true hyperthyroidism during pregnancy 1

Treatment Algorithm

Medication Management

• First trimester: Use propylthiouracil (PTU) due to lower risk of severe congenital malformations 1, 3
• Second and third trimesters: Switch to methimazole to avoid PTU-associated hepatotoxicity 1, 4
• Titrate to the lowest effective dose that maintains FT4 or FTI in the high-normal range 2, 1
• Monitor FT4 or FTI every 2-4 weeks to guide dosage adjustments 2
• Check TSH level once each trimester 1

Symptom Control

• Beta-blockers (e.g., propranolol) can be used temporarily to control symptoms until thioamide therapy reduces thyroid hormone levels 2, 1
• Note that hyperthyroidism may increase clearance of beta-blockers, potentially requiring higher doses until the patient becomes euthyroid 4

Monitoring and Follow-up

• Monitor for side effects of thioamide therapy 2:
  • Agranulocytosis (presents with sore throat and fever)
  • Hepatitis
  • Vasculitis
  • Thrombocytopenia
• If agranulocytosis symptoms develop, obtain a complete blood count and discontinue the thioamide immediately 2
• For patients on methimazole, monitor prothrombin time due to potential hypoprothrombinemia, especially before surgical procedures 4
• Rising serum TSH indicates that a lower maintenance dose of thioamide should be used 4

Fetal Considerations

• Monitor fetus for normal heart rate and appropriate growth 2
• Ultrasound screening for fetal goiter is not necessary unless problems are detected 2
• Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction 2
• Fetal thyroid is responsive to maternal antibodies and antithyroid drugs after 20 weeks' gestation 5

Special Situations

Thyroid Storm

• Thyroid storm is a medical emergency requiring immediate treatment 2, 1
• Treatment includes thioamides, potassium/sodium iodide solutions, dexamethasone, and supportive care 1

Surgery

• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy 2, 1
• Other indications for surgery include 6:
  • Allergy to antithyroid drugs
  • Large compressive goiter
  • Suspicion of thyroid cancer
  • Poor patient compliance
  • Extremely rare resistance to antithyroid drugs
• If necessary, surgery should be performed in the second trimester after achieving euthyroid state 6

Contraindicated Treatments

• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy 2, 1, 7

Postpartum Considerations

• Women treated with PTU or methimazole can breastfeed safely 2
• Methimazole is the preferred treatment during lactation 8
• Monitor thyroid function at frequent (weekly or biweekly) intervals during breastfeeding 4

Risks of Untreated Hyperthyroidism

• Maternal risks include 2, 1:
  • Severe preeclampsia
  • Preterm delivery
  • Heart failure
  • Miscarriage
• Fetal/neonatal risks include 2, 5:
  • Low birth weight
  • Stillbirth
  • Thyroid dysfunction