Management of Hyperthyroidism in Preterm Labor at 28 Weeks Gestation
Switch from methimazole to propylthiouracil (PTU) immediately, as methimazole is associated with congenital abnormalities and PTU is preferred in the first trimester, though switching to methimazole in the second and third trimesters is recommended—however, given the preterm labor emergency and severely uncontrolled hyperthyroidism (suppressed TSH, elevated FT4), continue methimazole at the current dose while managing the preterm labor, then transition to PTU if pregnancy continues. 1, 2
Immediate Thyroid Management Priorities
Medication Adjustment Strategy
The current methimazole dose (20mg BID = 40mg/day total) exceeds the safe upper limit for pregnancy. Methimazole up to 30 mg/day is considered safe in later pregnancy, so reduce the dose to 15mg BID (30mg/day maximum). 2
Continue propranolol 10mg BID for symptomatic control of tachycardia, tremor, and anxiety while awaiting thyroid hormone normalization. 2 Beta-blockers can be temporarily used until thioamide therapy reduces thyroid hormone levels. 1
The treatment goal is to maintain FT4 in the high-normal range or just above normal using the lowest effective thioamide dose. 2 The current FT4 of 2.05 ng/dL with suppressed TSH (0.0000 uIU/ml) indicates inadequate control requiring continued treatment.
Critical Monitoring Requirements
Check FT4 or free thyroxine index (FTI) every 2-4 weeks during active treatment until stable, then every 4 weeks once TSH stabilizes. 1, 2
A rising serum TSH indicates the need for dose reduction to prevent iatrogenic hypothyroidism. 2
Monitor complete blood count immediately if the patient develops sore throat, fever, or signs of infection (agranulocytosis warning). 2 Discontinue methimazole immediately if agranulocytosis is suspected. 2
Instruct the patient to promptly report new rash, hematuria, decreased urine output, dyspnea, or hemoptysis as these may indicate vasculitis. 2
Preterm Labor Management Considerations
Obstetric Priorities
Inadequately treated hyperthyroidism increases risks of preeclampsia, preterm delivery, heart failure, and possibly miscarriage. 1 The current uncontrolled thyroid status (suppressed TSH, elevated FT4) poses significant maternal and fetal risks.
Fetal and neonatal risks include low birth weight and potential thyroid dysfunction. 1 Maternal thyroid-stimulating antibodies (in Graves' disease) can cross the placenta and cause fetal/neonatal thyroid dysfunction. 2
Inform the neonatology team about maternal Graves' disease due to the risk of neonatal thyroid dysfunction from transplacental antibody transfer. 1, 2
Avoiding Thyroid Storm During Labor
Assess for thyroid storm using clinical criteria: fever, disproportionate tachycardia, altered mental status, vomiting, diarrhea, and cardiac arrhythmia. 1 This is a medical emergency requiring immediate treatment without delay for laboratory confirmation. 1
Avoid delivery during thyroid storm unless absolutely necessary. 1 If thyroid storm develops, treatment includes PTU or methimazole, potassium/sodium iodide solutions, dexamethasone, phenobarbital, and supportive care. 1
Monitor for cardiac dysfunction and heart failure given the severely suppressed TSH and elevated FT4, which increase cardiovascular risks during the stress of preterm labor.
Medication Safety Considerations
Beta-Blocker Dosing Adjustment
Hyperthyroidism causes increased clearance of beta-blockers with high extraction ratios. 3 As the patient becomes euthyroid with methimazole therapy, a dose reduction of propranolol may be needed. 3
Continue current propranolol dose (10mg BID) for now given the uncontrolled hyperthyroid state, but anticipate need for reduction as thyroid function normalizes.
Methimazole-Specific Warnings
Monitor prothrombin time before any surgical procedures (including potential cesarean delivery) as methimazole may cause hypoprothrombinemia and bleeding. 3
Watch for signs of hepatotoxicity: jaundice, pruritus, elevated bilirubin. 3 Methimazole can cause severe and reversible cholestatic jaundice in rare cases within the first few weeks of therapy. 4
Post-Delivery Planning
The newborn's physician must be informed about maternal Graves' disease to monitor for neonatal thyroid dysfunction. 1
Continue thyroid function monitoring postpartum as thyroid medication requirements frequently change after delivery.
If radioactive iodine treatment is considered postpartum, women must wait four months after I-131 treatment before breastfeeding. 1 However, I-131 is absolutely contraindicated during pregnancy. 1, 2