How to manage hyperthyroidism in pregnancy?

Management of Hyperthyroidism in Pregnancy

Treat hyperthyroidism in pregnancy with propylthiouracil (PTU) during the first trimester, then switch to methimazole for the second and third trimesters, using the lowest dose necessary to maintain free T4 in the high-normal range. 1, 2, 3

Initial Diagnosis and Assessment

• Confirm hyperthyroidism with TSH and free T4 or Free Thyroxine Index (FTI) testing in any pregnant woman with symptoms of thyrotoxicosis 4, 2
• Suppressed TSH with elevated T3 and T4 indicates true hyperthyroidism, most commonly from Graves' disease in pregnancy 1, 3
• Rule out gestational transient thyrotoxicosis (associated with hyperemesis gravidarum), which presents with biochemical hyperthyroidism but rarely requires treatment and resolves spontaneously 4, 1

The distinction matters: Only treat if clinical hyperthyroidism is present beyond nausea and vomiting alone, as hyperemesis-related thyrotoxicosis does not warrant antithyroid drugs. 4

Medication Management Algorithm

First Trimester (Weeks 0-13)

• Start propylthiouracil (PTU) as the preferred agent 1, 2, 3
• PTU is preferred despite its hepatotoxicity risk because methimazole carries a higher risk of congenital malformations (choanal atresia, aplasia cutis congenita, facial/cardiac/GI anomalies) during organogenesis 5, 6
• Use the lowest effective dose to maintain free T4 or FTI in the high-normal range 4, 1, 2

Second and Third Trimesters (Weeks 14-40)

• Switch from PTU to methimazole to reduce the risk of PTU-associated hepatotoxicity 1, 3, 5
• Continue using the lowest dose that maintains free T4 or FTI in the high-normal range 4, 1, 2
• The FDA warns that PTU can cause severe liver injury and acute liver failure, sometimes requiring transplantation, making the switch after the first trimester critical 6

Monitoring Schedule

• Check free T4 or FTI every 2-4 weeks to guide dosage adjustments 4, 1, 2
• Once stable, check TSH level once each trimester 4, 1, 2
• Monitor for medication side effects: agranulocytosis (presents with sore throat and fever), hepatitis, vasculitis, and thrombocytopenia 4

Critical pitfall: If sore throat and fever develop, obtain a complete blood count immediately and discontinue the thioamide due to risk of agranulocytosis. 4

Symptom Control

• Use a beta-blocker (e.g., propranolol) temporarily to control tachycardia, tremor, and anxiety until thioamide therapy reduces thyroid hormone levels 4, 1
• Beta-blockers provide symptomatic relief but do not treat the underlying hyperthyroidism 4

Fetal Monitoring

• Monitor fetal heart rate and growth to ensure normal development 4, 1, 2
• Ultrasound screening for fetal goiter is not necessary unless problems are detected 4, 1
• Although thioamides cross the placenta and can suppress fetal thyroid function, this is usually transient and rarely requires treatment 4
• Inform the newborn's physician about maternal Graves' disease because of the risk of neonatal thyroid dysfunction from transplacental passage of thyroid-stimulating antibodies 4, 1, 2

Risks of Untreated Hyperthyroidism

Untreated or inadequately treated maternal hyperthyroidism significantly increases risks of: 1, 2, 3

• Severe preeclampsia
• Preterm delivery
• Maternal heart failure
• Miscarriage and stillbirth
• Low birth weight

These maternal and fetal risks justify aggressive treatment despite medication concerns. 1, 2

Thyroid Storm Management

• Thyroid storm is a life-threatening emergency with high risk of maternal heart failure 4, 1
• Diagnose based on: fever, tachycardia out of proportion to fever, altered mental status (nervousness, restlessness, confusion, seizures), vomiting, diarrhea, and cardiac arrhythmia 4
• Do not delay treatment for laboratory confirmation 4

Treatment protocol:

• Propylthiouracil or methimazole 4, 1
• Saturated solution of potassium iodide or sodium iodide (alternatives: Lugol's solution, lithium) 4
• Dexamethasone 4
• Phenobarbital 4
• General supportive measures: oxygen, antipyretics, appropriate monitoring 4
• Avoid delivery during thyroid storm unless absolutely necessary 4

Alternative Treatments

• Thyroidectomy should be reserved only for women who do not respond to thioamide therapy 4, 1, 2
• If surgery is necessary, perform preferably during the second trimester 4
• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it ablates the fetal thyroid after 10 weeks gestation 4, 1, 2

Postpartum and Breastfeeding

• Women treated with either PTU or methimazole can safely breastfeed 4, 2, 3
• Do not breastfeed for four months after I-131 treatment if radioactive iodine was used postpartum 4, 2
• Monitor for postpartum thyroiditis with TSH and free T4 testing if goiter develops or symptoms of thyroid dysfunction appear 4

Key Clinical Pitfalls to Avoid

• Failing to switch from PTU to methimazole after the first trimester increases hepatotoxicity risk 3, 6
• Using methimazole in the first trimester increases risk of congenital malformations 5
• Over-treating with excessive thioamide doses can cause fetal hypothyroidism and goiter 4
• Not monitoring thyroid function every 2-4 weeks may result in inadequate control or over-treatment 4, 1, 3
• Continuing radioactive iodine therapy if pregnancy is discovered causes fetal thyroid ablation 4, 2