What is the management approach for a pregnant woman with hyperthyroidism (low TSH)?

Management of Low TSH in Pregnancy

Pregnant women with hyperthyroidism (low TSH) should be treated with propylthiouracil during the first trimester, then switched to methimazole for the second and third trimesters, with the goal of maintaining free T4 in the high-normal range using the lowest possible dose. 1

Initial Assessment and Diagnosis

When encountering low TSH in pregnancy, first distinguish between true hyperthyroidism requiring treatment versus physiologic conditions:

• Confirm diagnosis by measuring TSH along with free T4 or free thyroxine index (FTI), not total thyroid hormones which are elevated normally in pregnancy 1
• Evaluate for Graves' disease versus gestational transient thyrotoxicosis, as treatment differs significantly 2
• Screen for hyperemesis gravidarum, which causes biochemical hyperthyroidism (undetectable TSH, elevated FTI) but rarely requires treatment unless other clinical signs of hyperthyroidism are present 1

Antithyroid Drug Selection by Trimester

The choice of thioamide medication is trimester-specific due to differing teratogenic risks:

First Trimester (Weeks 1-13)

• Use propylthiouracil (PTU) exclusively during organogenesis 1, 3
• Methimazole carries risk of congenital malformations including aplasia cutis and other anomalies during first trimester exposure 3, 2
• PTU has less severe teratogenicity compared to methimazole 2, 4

Second and Third Trimesters (Weeks 14-40)

• Switch to methimazole for remainder of pregnancy 1, 3
• This switch is recommended because PTU carries significant hepatotoxicity risk with continued use 1, 3
• Methimazole is the preferred antithyroid drug outside the first trimester 3

Treatment Targets and Monitoring

Target free T4 or FTI in the high-normal range using the lowest possible thioamide dose to minimize fetal thyroid suppression 1, 5:

• Measure free T4 or FTI every 2-4 weeks until stable 1
• Trimester-specific TSH reference ranges: 0.1-2.5 mIU/L (first), 0.2-3.0 mIU/L (second), 0.3-3.0 mIU/L (third) 6
• Doses often decrease in third trimester due to immune-suppressant effects of pregnancy 6

Symptomatic Management

Until thioamide therapy reduces thyroid hormone levels:

• Add beta-blocker (propranolol) to control tachycardia and other hyperadrenergic symptoms 1
• Monitor for dose reduction needs as patient becomes euthyroid, since hyperthyroidism increases beta-blocker clearance 3

Critical Monitoring for Complications

Maternal Monitoring

• Watch for agranulocytosis: presents with sore throat and fever; obtain CBC immediately and discontinue thioamide if suspected 1
• Other thioamide side effects include hepatitis, vasculitis, and thrombocytopenia 1
• Assess for thyroid storm risk: fever, tachycardia disproportionate to fever, altered mental status, vomiting, diarrhea, cardiac arrhythmia—this is a medical emergency requiring immediate multi-drug therapy 1

Fetal Monitoring

• Check for fetal thyrotoxicosis in women with current or past Graves' disease, as TSH-receptor antibodies cross the placenta 1, 7
• Serial ultrasounds to detect fetal goiter or other indirect signs of fetal thyroid dysfunction 7
• Transient fetal/neonatal thyroid suppression can occur with thioamide therapy but is usually self-limited 1

Risks of Inadequate Control

Untreated or inadequately treated hyperthyroidism increases risk of:

• Maternal complications: severe preeclampsia, preterm delivery, heart failure, miscarriage 1
• Fetal/neonatal complications: low birth weight, fetal thyrotoxicosis, neonatal hyperthyroidism 3, 6

Special Considerations

• Breastfeeding is safe with both propylthiouracil and methimazole; monitor infant thyroid function at frequent intervals 1, 3
• Avoid radioactive iodine entirely during pregnancy; if inadvertently exposed after first trimester, consider pregnancy continuation risks 1
• Women should delay pregnancy at least 6 months after radioactive iodine treatment 2