What is the management of hyperthyroidism (excessive production of thyroid hormone) in a pregnant woman?

Management of Hyperthyroidism in Pregnancy

First-Line Medication Strategy

Propylthiouracil (PTU) is the preferred antithyroid medication during the first trimester of pregnancy, with a switch to methimazole recommended for the second and third trimesters. 1, 2

First Trimester (Weeks 1-13)

• Use PTU as the primary antithyroid drug due to lower risk of congenital abnormalities compared to methimazole during organogenesis 1, 3, 4
• Methimazole and carbimazole are associated with a specific pattern of birth defects including choanal atresia, aplasia cutis congenita, and facial, cardiac, gastrointestinal, and skin anomalies 3, 4
• PTU carries a small but serious risk of hepatotoxicity that can progress to liver failure requiring transplantation, though this is rare 4, 5

Second and Third Trimesters (Weeks 14-40)

• Switch from PTU to methimazole after the first trimester to minimize maternal hepatotoxicity risk 2, 4
• Methimazole is preferred in later pregnancy because the critical period for major congenital malformations has passed 2, 4

Treatment Goals and Monitoring

Target Thyroid Levels

• Maintain maternal free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible thioamide dosage 1
• Measure free T4 and free T3 rather than total hormone levels, as total T4 and T3 are physiologically elevated in pregnancy due to increased thyroxine-binding globulin 6

Monitoring Schedule

• Check thyroid function (free T4 or FTI) every 2-4 weeks to guide dosage adjustments 1
• Monitor TSH every trimester once stable 1
• As pregnancy progresses, thyroid dysfunction often diminishes, allowing dose reduction or even discontinuation several weeks to months before delivery 2

Adjunctive Symptomatic Management

• Use beta-adrenergic blockers (propranolol) temporarily to control symptoms like tremors, palpitations, and tachycardia while awaiting thioamide effect 1
• Beta-blockers, thyrotropin-releasing hormone, thyroid-stimulating immunoglobulins, antithyroid drugs, iodides, and beta-blockers all cross the placenta readily 6

Critical Safety Monitoring

Maternal Monitoring

• Monitor for agranulocytosis—immediately discontinue thioamide if sore throat and fever develop 1
• Watch for signs of hepatotoxicity, particularly with PTU (jaundice, dark urine, abdominal pain, nausea) 4, 5
• Monitor prothrombin time before surgical procedures, as methimazole may cause hypoprothrombinemia and bleeding 2

Fetal Considerations

• By 20 weeks' gestation, the fetal thyroid is fully responsive to both thyroid-stimulating immunoglobulins and antithyroid drugs 6
• Antithyroid drugs cross the placenta and can induce fetal goiter and hypothyroidism if maternal dosing is excessive 2, 6
• Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction 1

Surgical Management (Thyroidectomy)

Reserve thyroidectomy for specific indications only:

• Failure to respond to thioamide therapy 1
• Intolerance to antithyroid drugs (agranulocytosis or severe hepatotoxicity) 1
• Life-threatening, uncontrollable hyperthyroidism 5

If surgery is necessary, perform during the second trimester 1

Absolute Contraindications

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

Risks of Inadequate Treatment

Maternal Complications

• Preeclampsia 1
• Maternal heart failure 2, 7
• Preterm delivery 1, 4

Fetal and Neonatal Complications

• Spontaneous abortion/miscarriage 2, 6
• Stillbirth 2, 6
• Low birth weight 1
• Fetal or neonatal hyperthyroidism 2
• Increased risk of congenital anomalies (possibly related to uncontrolled maternal hyperthyroidism itself) 4

Special Clinical Scenarios

Hyperemesis Gravidarum

• Biochemical hyperthyroidism associated with hyperemesis gravidarum (gestational transient thyrotoxicosis) is mediated by high human chorionic gonadotropin levels 7
• This condition rarely requires treatment unless other clinical signs of hyperthyroidism are present 1
• Routine thyroid testing is not recommended unless hyperthyroid signs exist beyond hyperemesis 1

Thyroid Storm

• Thyroid storm is a medical emergency presenting with fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia 1
• Begin treatment immediately without waiting for laboratory confirmation 1
• Treatment includes PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care 1
• Avoid delivery during thyroid storm unless absolutely necessary 1

Breastfeeding Considerations

• Methimazole is present in breast milk but multiple studies found no adverse effects on nursing infants 2
• A long-term study of 139 thyrotoxic lactating mothers demonstrated no toxicity in breastfed infants 2
• Monitor infant thyroid function at frequent (weekly or biweekly) intervals if mother is taking antithyroid medication 2
• Both PTU and methimazole enter breast milk in small amounts 3

Common Pitfalls to Avoid

• Do not use total T4 and T3 levels for diagnosis—these are physiologically elevated in pregnancy; use free T4 and free T3 instead 6
• Do not continue PTU throughout pregnancy—switch to methimazole after the first trimester to reduce maternal hepatotoxicity risk 2, 4
• Do not use excessive antithyroid drug doses—this increases risk of fetal hypothyroidism and goiter 2, 6
• Do not overlook the need for dose reduction as pregnancy progresses—thyroid dysfunction often improves in later pregnancy 2