What is the best course of action for a 31-week pregnant woman with uncontrolled thyrotoxicosis, currently taking 10mg of carbimazole (generic name), experiencing palpitations and anxiety, with thyroid function tests showing suppressed Thyroid-Stimulating Hormone (TSH) and elevated Triiodothyronine (T3) and Thyroxine (T4) levels?

Increase Carbimazole Dose Immediately

For a 31-week pregnant woman with uncontrolled thyrotoxicosis on 10mg carbimazole, the correct course of action is to increase the carbimazole dose (Option A). This approach maintains the current medication while achieving better thyroid control, which is critical for both maternal and fetal outcomes at this gestational age.

Rationale for Dose Escalation

• The goal during pregnancy is to maintain free T4 or free T3 in the high-normal range using the lowest possible thioamide dosage, with monitoring every 2-4 weeks 1
• Current symptoms (palpitations, anxiety) combined with suppressed TSH and elevated T3/T4 indicate inadequate disease control requiring immediate intervention 1
• Untreated or inadequately controlled thyrotoxicosis during pregnancy increases risk of maternal complications (heart failure, preeclampsia), obstetrical complications (miscarriage, stillbirth, preterm labor), and fetal complications 2, 3

Why Not Switch to Propylthiouracil (Option B)

• Switching antithyroid drugs at 31 weeks gestation is unnecessary and potentially disruptive to established therapy 1
• PTU is preferred in the first trimester due to lower teratogenic risk, but after the first trimester, carbimazole can be safely continued 2, 4
• The recent reassessment of teratogenic effects primarily concerns first-trimester exposure, which is no longer relevant at 31 weeks 2
• Both drugs cross the placenta equally and can affect fetal thyroid function, so switching provides no advantage at this stage 3

Why Not Surgical Intervention (Option C)

• Thyroidectomy should be reserved for women who do not respond to thioamide therapy or have severe drug allergies 1
• Surgery during pregnancy carries increased risks of preterm labor and miscarriage 5
• Partial thyroidectomy is preferably performed in the second trimester if absolutely necessary, not in the third trimester at 31 weeks 1
• This patient has not yet failed medical management—she simply needs dose optimization 6

Why Not Radioiodine (Option D)

• Treatment with iodine-131 is absolutely contraindicated in pregnant women 1
• After 10 weeks gestation, radioiodine exposure can ablate the fetal thyroid, causing congenital hypothyroidism 1
• At 31 weeks, the fetal thyroid is fully functional and highly vulnerable to radioiodine damage 3

Why Not Emergency Delivery (Option E)

• Delivery at 31 weeks would result in significant prematurity with associated neonatal complications 1
• Thyrotoxicosis alone, even when uncontrolled, does not mandate emergency delivery unless thyroid storm develops 1
• Medical optimization should be attempted first, as most cases respond to appropriate antithyroid drug dosing 2, 4
• Unless deemed necessary for maternal or fetal indications, delivery during active thyrotoxicosis should be avoided 1

Practical Management Algorithm

Immediate actions:

• Increase carbimazole dose incrementally (typically to 15-20mg daily, divided doses) 6
• Add propranolol for symptomatic relief of palpitations and anxiety until thyroid hormones normalize 1
• Monitor free T4 or free T3 every 2-4 weeks to guide further dose adjustments 1

Monitoring requirements:

• Check for normal maternal heart rate and appropriate fetal growth 1
• Ultrasound screening for fetal goiter is not necessary unless specific problems are detected 1
• Measure TRAb (thyroid-stimulating hormone receptor antibodies) if not already done, as this helps predict risk of fetal/neonatal thyrotoxicosis 4

Target thyroid function:

• Maintain free T4 in the high-normal range to minimize fetal thyroid suppression while controlling maternal disease 1
• TSH will remain suppressed and should not be used as the primary monitoring parameter 1

Critical Safety Considerations

• Although suppression of fetal and neonatal thyroid function can occur with thioamide therapy, it is usually transient and treatment is rarely required 1
• The newborn's physician must be informed that the mother has Graves' disease due to associated risk of neonatal thyroid dysfunction 1
• Watch for agranulocytosis symptoms (sore throat, fever) which would require immediate CBC and drug discontinuation 1
• At 31 weeks, the fetal thyroid is fully responsive to both maternal antibodies and antithyroid drugs, requiring careful dose titration 3

Common Pitfall to Avoid

The most critical error would be switching to PTU or pursuing definitive therapy (surgery/radioiodine) without first attempting carbimazole dose optimization. Most cases of apparent "drug resistance" actually represent inadequate dosing rather than true pharmacologic resistance 6. Only after documented failure of appropriate doses (up to 40-60mg daily if needed) should alternative therapies be considered 6.