In a pregnant patient with hyperthyroidism, how frequently should repeat thyroid function tests (thyroid‑stimulating hormone and free thyroxine) be performed?

Monitoring Frequency for Thyroid Function Tests in Pregnant Women with Hyperthyroidism

In pregnant women with hyperthyroidism, thyroid function tests (free T4 or free thyroxine index) should be checked every 2–4 weeks to guide medication dose adjustments and maintain optimal maternal and fetal outcomes. 1

Monitoring Protocol During Pregnancy

Active Hyperthyroidism on Antithyroid Drugs

• Check free T4 (or FTI) every 2–4 weeks throughout pregnancy while the patient is on propylthiouracil (first trimester) or methimazole (second and third trimesters) 1
• The goal is to maintain free T4 in the high-normal range using the lowest possible thioamide dose, which prevents both maternal complications and fetal thyroid suppression 1
• Once thyroid hormone levels stabilize on a consistent medication dose, TSH should be checked every trimester 1

Rationale for Frequent Monitoring

The 2–4 week interval is critical because:

• Thyroid hormone requirements change dynamically throughout pregnancy due to physiologic alterations in thyroid-binding globulin, hCG stimulation, and increased metabolic demands 2, 3
• Antithyroid medications require dose titration to balance maternal control against the risk of fetal hypothyroidism 1
• Inadequate treatment increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 1

Trimester-Specific Considerations

First Trimester

• Use propylthiouracil exclusively during weeks 0–13 to minimize congenital malformations 1
• Monitor free T4 every 2–4 weeks and adjust PTU dose accordingly 1

Second and Third Trimesters

• Switch to methimazole after the first trimester to reduce maternal hepatotoxicity risk 1
• Continue monitoring free T4 every 2–4 weeks 1
• Once stable, TSH can be checked each trimester 1

Additional Monitoring Requirements

Maternal Safety

• Watch for agranulocytosis (sore throat, fever) – obtain immediate CBC and discontinue thioamide if confirmed 1
• Monitor for hepatotoxicity, vasculitis, and thrombocytopenia as potential drug-related toxicities 1

Fetal Surveillance

• In women with Graves' disease, monitor fetal heart rate and growth throughout pregnancy 1
• Fetal thyroid ultrasound is valuable when maternal TSH receptor antibodies are elevated ≥5 times the upper normal limit, as this predisposes to fetal hyperthyroidism 4
• Abnormal fetal thyroid sonogram may be the only sign of fetal thyroid dysfunction 4

Special Circumstances

Hyperemesis Gravidarum

• Biochemical hyperthyroidism associated with hyperemesis gravidarum rarely requires treatment unless other clinical signs of hyperthyroidism are present 1

Thyroid Storm

• This medical emergency requires immediate treatment with PTU or methimazole, potassium/sodium iodide, dexamethasone, phenobarbital, and supportive care without waiting for laboratory confirmation 1
• Delivery should be avoided during active thyroid storm unless absolutely necessary 1

Common Pitfalls to Avoid

• Never target mid-normal or low-normal free T4 levels – maintaining high-normal range reduces fetal hypothyroidism risk 1
• Do not use radioactive iodine during pregnancy as it causes fetal thyroid ablation 1
• Failing to switch from PTU to methimazole after the first trimester increases maternal hepatotoxicity risk 1
• Do not dismiss monitoring frequency – the 2–4 week interval is essential for dose optimization and preventing both maternal and fetal complications 1

Postpartum Monitoring

• Inform the newborn's physician about maternal hyperthyroidism due to risk of neonatal thyroid dysfunction 1
• Women treated with PTU or methimazole can safely breastfeed 1
• Monitor for postpartum thyroiditis in women with history of thyroid dysfunction 1