Drug of Choice for Hyperthyroidism in Pregnancy

The drug of choice is propylthiouracil (PTU) for this pregnant woman presenting with Graves' disease in her first trimester. 1

Rationale for PTU in First Trimester

PTU is preferred during the first trimester of pregnancy (organogenesis period) because methimazole is associated with rare but serious congenital malformations. 2, 1
The FDA drug label explicitly states that "it may be appropriate to use an alternative anti-thyroid medication in pregnant women requiring treatment for hyperthyroidism particularly in the first trimester of pregnancy during organogenesis," making PTU the preferred choice initially. 2
Methimazole has been linked to rare fetal abnormalities including aplasia cutis, choanal atresia, and esophageal atresia, which occur during first trimester exposure. 1, 3

Immediate Management:

Start PTU at the lowest effective dose to maintain maternal free T4 at the high-normal range or slightly above normal (not excessively suppressed). 4, 5
The goal is to control maternal hyperthyroidism while minimizing fetal exposure to antithyroid medication. 4
Add propranolol temporarily to control acute symptoms (tachycardia, tremors, palpitations) until PTU reduces thyroid hormone levels. 4

Monitoring Requirements:

Monitor free T4 or free thyroxine index (FTI) every 2-4 weeks initially to adjust PTU dosage. 4
Check complete blood count and liver function tests before starting and periodically during therapy due to risks of agranulocytosis and hepatotoxicity. 1
Measure thyrotropin receptor antibodies (TRAb) levels, as this patient likely has Graves' disease based on diffuse goiter and proptosis. 4, 6

Second and Third Trimesters:

Consider switching from PTU to methimazole after the first trimester is complete because PTU carries significant risk of maternal hepatotoxicity, including hepatic failure. 2, 1
The FDA labels for both drugs state: "Given the potential maternal adverse effects of propylthiouracil (e.g., hepatotoxicity), it may be preferable to switch from propylthiouracil to methimazole for the second and third trimesters." 2, 1
Many pregnant women experience amelioration of hyperthyroidism as pregnancy progresses, allowing dose reduction or even discontinuation in some cases. 4, 3

Never use radioactive iodine (RAI) during pregnancy - it is absolutely contraindicated as it crosses the placenta and ablates the fetal thyroid. 7
Thyroidectomy should be reserved only for extreme cases where medical therapy fails or severe adverse reactions occur, and should be limited to the second trimester if absolutely necessary. 7
Do not over-treat - excessive antithyroid medication can cause fetal hypothyroidism and goiter, which is why the target is high-normal maternal free T4, not complete normalization. 5, 3
Monitor for agranulocytosis - instruct the patient to immediately report fever, sore throat, or pharyngitis and discontinue PTU if these occur. 1
Watch for hepatotoxicity signs - nausea, anorexia, tiredness, fever, or malaise warrant immediate PTU discontinuation and liver function testing. 1

If TRAb levels are elevated (which is likely given the clinical presentation), continued treatment throughout pregnancy is necessary to prevent fetal and neonatal thyrotoxicosis. 4
Maternal antibodies cross the placenta and can stimulate the fetal thyroid even if the mother is euthyroid on medication. 4, 5
Inform the newborn's physician about maternal Graves' disease due to risk of neonatal thyroid dysfunction in the first days to months of life. 4, 6

Methimazole (Option C) would be second-line during first trimester due to teratogenic risk, though it becomes preferred after organogenesis. 2
RAI (Option A) is absolutely contraindicated in pregnancy. 7
Thyroidectomy (Option D) is reserved only for severe cases unresponsive to medical therapy or with severe drug reactions. 7