How to manage pregnancy-induced hyperthyroidism in a female patient of childbearing age with a history of thyroid disease?

Management of Pregnancy-Induced Hyperthyroidism

Use propylthiouracil (PTU) as first-line treatment throughout the first trimester, then switch to methimazole for the second and third trimesters to minimize both congenital abnormalities and maternal hepatotoxicity risks. 1, 2

First Trimester Management (Weeks 0-13)

Start PTU immediately as it crosses the placenta minimally and carries lower risk of fetal abnormalities compared to methimazole, which has been associated with rare but serious congenital malformations during organogenesis (weeks 6-10). 1, 2, 3

Treatment Goals

• Maintain free T4 or free thyroxine index (FTI) in the high-normal range or just above normal using the lowest effective dose 1, 2
• Monitor free T4 or FTI every 2-4 weeks during active treatment until stable 1, 2
• Avoid overtreatment—a rising TSH indicates need for dose reduction 2

Symptomatic Management

• Add propranolol temporarily for tachycardia, tremors, and anxiety while awaiting thyroid hormone normalization 1, 2
• Beta-blockers provide rapid symptom relief but address only symptoms, not the underlying hyperthyroidism 1

Second and Third Trimester Management (Weeks 14-40)

Switch from PTU to methimazole (up to 30 mg/day) for the remainder of pregnancy, as PTU carries significant hepatotoxicity risk with prolonged use. 1, 2, 3

Ongoing Monitoring

• Continue checking free T4 or FTI every 2-4 weeks until stable 1, 2
• Once TSH stabilizes, monitor every 4 weeks 2
• Many women experience spontaneous improvement in hyperthyroidism as pregnancy progresses, allowing dose reduction or even discontinuation weeks to months before delivery 3

Critical Safety Monitoring Throughout Pregnancy

Immediate Reporting Instructions

Instruct patients to immediately report the following warning signs: 2, 3

• Sore throat or fever (agranulocytosis—obtain CBC immediately and discontinue drug if suspected)
• New rash, hematuria, decreased urine output (vasculitis)
• Dyspnea or hemoptysis (pulmonary complications)

Additional Monitoring

• Watch for hepatitis, thrombocytopenia, and vasculitis 2
• Monitor prothrombin time before surgical procedures (methimazole may increase anticoagulant effects) 3

Special Clinical Scenarios

Hyperemesis Gravidarum

• Biochemical hyperthyroidism with hyperemesis gravidarum rarely requires treatment unless other clinical signs of hyperthyroidism are present 1
• Do not routinely screen for thyroid dysfunction in hyperemesis unless hyperthyroid symptoms exist 1

Thyroid Storm (Medical Emergency)

Presents with fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia. 1

Immediate treatment without waiting for lab confirmation: 1

• PTU or methimazole
• Potassium/sodium iodide solutions
• Dexamethasone
• Phenobarbital
• Supportive care
• Avoid delivery during thyroid storm unless absolutely necessary 1

Surgical Management

Reserve thyroidectomy for: 1, 2

• Patients who fail medical therapy
• Large compressive goiters
• Intolerance to antithyroid drugs (agranulocytosis or severe hepatotoxicity)

If surgery is necessary, perform during the second trimester when safest for both mother and fetus. 1, 2

Absolute Contraindications

Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation. 1, 2, 4

Risks of Inadequate Treatment

Untreated or inadequately treated hyperthyroidism significantly increases risk of: 1, 3, 5

• Preeclampsia
• Preterm delivery
• Maternal heart failure
• Spontaneous abortion and stillbirth
• Low birth weight
• Fetal/neonatal thyroid dysfunction

Fetal Considerations

• Maternal thyroid-stimulating antibodies (in Graves' disease) cross the placenta and can cause fetal/neonatal hyperthyroidism 2, 4
• Inform the pediatrician of maternal Graves' disease before delivery due to risk of neonatal thyroid dysfunction 1, 2, 4
• Fetal thyroid suppression from thioamides is usually transient and rarely requires treatment 4

Postpartum and Lactation

Methimazole (up to 30 mg/day) is now preferred over PTU during lactation due to severe hepatotoxicity concerns with prolonged PTU use, despite PTU transferring only 0.025% into breast milk. 4

• Women can safely breastfeed while taking methimazole 4
• If radioactive iodine treatment is given postpartum, wait 4 months before breastfeeding 1, 4