What medication should be given to a 14-week pregnant patient with primary hyperthyroidism?

Treatment of Primary Hyperthyroidism at 14 Weeks Gestation

Propylthiouracil (PTU) is the preferred medication for treating primary hyperthyroidism in a 14-week pregnant patient, as it carries a lower risk of congenital malformations during the first trimester compared to methimazole. 1, 2

Medication Selection and Rationale

• PTU should be used during the first trimester (through approximately 16 weeks) because methimazole is associated with rare but specific congenital defects including aplasia cutis, choanal atresia, esophageal atresia, and craniofacial malformations when used during organogenesis 1, 3, 4

• At 14 weeks gestation, you are still within the critical window where PTU is preferred, though organogenesis is largely complete 1

• Consider switching to methimazole after the first trimester (after 16 weeks) for the remainder of pregnancy, as PTU carries a risk of severe maternal hepatotoxicity including hepatic failure requiring liver transplantation or death 1, 2, 4

• The evidence shows no significant differences in fetal thyroid function between PTU and methimazole in cord blood samples, but the teratogenic risk profile differs by trimester 5

Dosing and Treatment Goals

• Start PTU at 100-300 mg divided every 6 hours (typical starting dose) 6

• The treatment goal is to maintain free T4 or free thyroxine index (FTI) in the high-normal range using the lowest possible dose to minimize fetal thyroid suppression 5, 1

• Use the minimum effective dose because PTU crosses the placenta and can induce fetal goiter and cretinism if excessive 5, 2

Monitoring Requirements

• Check free T4 or FTI every 2-4 weeks to adjust dosage and ensure adequate but not excessive treatment 5, 1

• Monitor TSH every trimester once stable 5, 1

• If the patient develops sore throat, fever, tiredness, nausea, or anorexia, immediately discontinue PTU and obtain a complete blood count to evaluate for agranulocytosis, and check liver function tests for hepatotoxicity 5, 2

• Monitor prothrombin time before any surgical procedures, as PTU can cause hypoprothrombinemia 2

Adjunctive Symptomatic Management

• A beta-blocker such as propranolol can be used temporarily to control symptoms like tremors, palpitations, and tachycardia until the thioamide reduces thyroid hormone levels 5, 1

• Beta-blockers should be used cautiously and only for symptom control, not as primary therapy 5

Critical Safety Warnings

• Radioactive iodine (I-131) is absolutely contraindicated during pregnancy as it causes fetal thyroid ablation after 10 weeks gestation 5, 1

• Untreated or inadequately treated hyperthyroidism increases risks of preeclampsia, preterm delivery, stillbirth, maternal heart failure, and low birth weight 5, 1, 2

• Watch for PTU-induced hepatotoxicity: if liver transaminases exceed 3 times the upper limit of normal or clinical signs of liver dysfunction appear (anorexia, pruritus, right upper quadrant pain), discontinue PTU immediately 2, 3

• Watch for agranulocytosis: this life-threatening complication typically presents with sore throat and fever; if suspected, stop PTU immediately and check white blood cell count 5, 2

Special Considerations for This Patient

• Many pregnant women experience improvement in thyroid dysfunction as pregnancy progresses, potentially allowing dose reduction or even discontinuation in later pregnancy 5, 2

• Inform the newborn's physician about maternal Graves' disease because of the risk of neonatal thyroid dysfunction from transplacental passage of thyroid-stimulating immunoglobulins 5, 1

• Thyroidectomy should be reserved only for patients who do not respond to thioamide therapy or develop severe drug intolerance; if surgery is necessary, the second trimester is preferred 1

• The patient can safely breastfeed while taking PTU, as it is present in breast milk in clinically insignificant amounts 5, 2