What is the management of hyperthyroidism (excessive production of thyroid hormone) in pregnancy?

Management of Hyperthyroidism in Pregnancy

Thioamides (propylthiouracil or methimazole) are the first-line medications for managing hyperthyroidism in pregnancy, with propylthiouracil preferred in the first trimester and methimazole recommended for the second and third trimesters. 1

Diagnosis and Evaluation

• Initial screening should include TSH, Free T4 (FT4) or Free T4 Index (FTI) in pregnant women with suspected hyperthyroidism 1
• Differentiate between causes of hyperthyroidism:
  • Graves' disease (most common cause of clinically significant hyperthyroidism)
  • Gestational transient thyrotoxicosis (associated with hyperemesis gravidarum)
  • Other causes (toxic multinodular goiter, solitary autonomous nodule) 2

Treatment Algorithm

First Trimester

1. Use propylthiouracil (PTU) as first-line therapy due to lower risk of congenital malformations compared to methimazole 1
2. Use the lowest possible dose to maintain FT4 in the high-normal range 1
3. Monitor FT4 or FTI every 2-4 weeks to adjust dosing 1

Second and Third Trimesters

1. Consider switching from PTU to methimazole after the first trimester due to lower risk of hepatotoxicity with methimazole 1, 3
2. Continue using the lowest effective dose to maintain FT4 in high-normal range 1
3. Continue monitoring FT4 every 2-4 weeks 1

Adjunctive Therapy

• Beta-blockers (e.g., propranolol) can be used temporarily for symptomatic relief until thioamide therapy reduces thyroid hormone levels 1
• In thyroid storm (extreme hypermetabolic state), immediate intervention with thioamides, saturated solution of potassium iodide or sodium iodide, beta-blockers, and supportive care is required 1

Special Considerations

Gestational Transient Thyrotoxicosis

• Usually requires only supportive care and hydration 1
• No routine thyroid testing unless other signs of hyperthyroidism are present 1

Medication Side Effects and Monitoring

• Monitor for agranulocytosis (presents with sore throat and fever), hepatitis, vasculitis, and thrombocytopenia as potential side effects of thioamide therapy 1, 3
• Monitor prothrombin time during methimazole therapy, especially before surgical procedures, due to potential hypoprothrombinemia 3

Thyroid Receptor Antibodies

• Should be assessed in all women with hyperthyroidism to help predict risk of fetal or neonatal thyroid dysfunction 4

Drug Interactions

• Methimazole may increase the activity of oral anticoagulants through inhibition of vitamin K activity 3
• Hyperthyroidism increases clearance of beta-blockers; dose reduction may be needed when patient becomes euthyroid 3
• Serum digitalis levels may increase when hyperthyroid patients become euthyroid 3

Postpartum Considerations

• Hyperthyroidism may recur in the postpartum period as Graves' disease or postpartum thyroiditis 4
• Evaluate thyroid function 6 weeks after delivery 4
• Both propylthiouracil and methimazole are considered safe during breastfeeding 1
• Propranolol is preferred during breastfeeding as it accumulates less in breast milk 1

Contraindicated Treatments

• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy 1
• Thyroidectomy should be reserved for women who do not respond to thioamide therapy 1
• Surgical thyroidectomy during pregnancy with active disease may lead to isolated fetal hyperthyroidism 5

Important Caveats

• Inadequate or overly aggressive antithyroid drug therapy can harm the fetus 5
• Antithyroid drugs tend to block fetal thyroid function more effectively than maternal thyroid function 5
• Maternal thyroid hormone levels should be kept in the upper third of the reference range or just above normal 4
• Untreated or inadequately treated hyperthyroidism increases risk of maternal heart failure, spontaneous abortion, preterm birth, stillbirth, and fetal/neonatal hyperthyroidism 3
• In many pregnant women, thyroid dysfunction diminishes as pregnancy proceeds, allowing for dose reduction 3