How do you manage hyperthyroidism in pregnant patients?

Management of Hyperthyroidism in Pregnancy

Use propylthiouracil (PTU) as first-line treatment during the first trimester, then consider switching to methimazole for the second and third trimesters, maintaining maternal free T4 in the high-normal range with the lowest effective dose. 1, 2

First-Line Pharmacologic Approach

First Trimester Strategy

• PTU is the preferred agent during the first trimester because it crosses the placenta minimally (only 0.025% into breast milk) compared to methimazole, and methimazole has been associated with rare congenital malformations during organogenesis 1, 2
• PTU should be reserved for patients who cannot tolerate methimazole, except during the first trimester of pregnancy when it becomes the preferred choice 2

Second and Third Trimester Considerations

• Switch from PTU to methimazole after the first trimester (up to 30 mg/day is considered safe) to avoid PTU-associated maternal hepatotoxicity risk 1, 3
• The FDA warns that PTU has caused severe liver injury and acute liver failure, including cases requiring liver transplantation, making the switch to methimazole preferable after organogenesis 2

Treatment Goals and Monitoring Protocol

Target Thyroid Levels

• Maintain maternal FT4 or FTI in the high-normal range or just above normal using the lowest effective thioamide dose to prevent fetal hypothyroidism and goiter 1, 4
• Aim for high normal or slightly elevated thyroid function in the mother rather than complete normalization 4

Monitoring Frequency

• Check FT4 or FTI every 2-4 weeks during active treatment until stable 1
• Once TSH level is stable, monitor every 4 weeks 1
• Follow patients at 3-week intervals if progress is satisfactory, more frequently if not 4
• Progressively reduce PTU dosage in anticipation of the customary amelioration in hyperthyroidism that occurs in later stages of pregnancy 4
• Approximately one-third of patients can discontinue antithyroid drugs in the second half of pregnancy 4

Critical Safety Monitoring

Immediate Reporting Requirements

• Instruct patients to immediately report sore throat, fever, or signs of infection and obtain complete blood count immediately if agranulocytosis is suspected 1
• Patients must promptly report new rash, hematuria, decreased urine output, dyspnea, or hemoptysis as these may indicate vasculitis 1, 3
• Monitor for hepatitis, vasculitis, and thrombocytopenia 1

Laboratory Monitoring

• Monitor prothrombin time before surgical procedures due to potential hypoprothrombinemia and bleeding risk 3
• Thyroid function tests should be monitored periodically during therapy 3

Thyroid Storm Management (Medical Emergency)

Acute Treatment Protocol

• Administer standard drug series: propylthiouracil or methimazole; saturated solution of potassium iodide or sodium iodide; dexamethasone; and phenobarbital 1
• Provide general supportive measures including oxygen, antipyretics, and appropriate monitoring 1
• Avoid delivery during thyroid storm unless absolutely necessary 1
• Evaluate fetal status with ultrasound, nonstress testing, or biophysical profile depending on gestational age 1
• Thyroid storm affects <1% of pregnant women with hyperthyroidism but is a medical emergency 5

Alternative Treatment Options

Surgical Thyroidectomy

• Reserve thyroidectomy for patients who fail medical therapy, have large compressive goiters, or strongly prefer surgery 1
• Perform surgery during the second trimester when safest 1

Contraindicated Treatments

• Radioactive iodine is absolutely contraindicated during pregnancy and lactation 1
• Women must not breastfeed for 4 months after I-131 treatment 1

Maternal and Fetal Risk Considerations

Untreated Hyperthyroidism Risks

• Pregnant women with inadequately treated hyperthyroidism face increased risk for severe preeclampsia, preterm delivery, heart failure, and possibly miscarriage 5
• Low birth weight in neonates can occur 5
• Increased risk of maternal heart failure, spontaneous abortion, preterm birth, stillbirth, and fetal or neonatal hyperthyroidism 3

Fetal Monitoring Concerns

• Consider fetal thyrotoxicosis in women with a history of Graves' disease because thyroid-stimulating antibodies cross the placenta 5
• Monitor for neonatal immune-mediated hypothyroidism or hyperthyroidism due to transplacental antibody passage 5
• Both maternal thyroid excess and antithyroid treatments may adversely affect newborn health 6

Postpartum Management

• Evaluate thyroid function 6 weeks after delivery to detect postpartum thyroiditis or Graves' disease recurrence 1
• Diagnose postpartum thyroiditis with new onset of abnormal TSH or FT4 levels 1
• Women treated with PTU or methimazole can breastfeed safely, though methimazole is present in breast milk 5, 3
• Long-term studies of 139 thyrotoxic lactating mothers found no toxicity in nursing infants receiving methimazole through breast milk 3

Prescription Management Pitfall

• Limit PTU prescriptions to only the amount required until the next scheduled visit because pregnant hyperthyroid patients may continue medication without supervision, risking fetal hypothyroidism 4
• After pregnancy termination, treat persistent or recurrent hyperthyroidism definitively to prevent another episode of pregnancy complicated by hyperthyroidism 4