Managing Thyroid Levels During Pregnancy
Pregnant women with hypothyroidism should maintain TSH levels within trimester-specific reference ranges, with monitoring at minimum during each trimester and more frequent testing (every 4-6 weeks) until TSH levels stabilize. 1
Screening and Monitoring Guidelines
Initial Assessment
- TSH is the recommended initial screening test for thyroid dysfunction in pregnant women with suspected thyroid disease 1
- Additional testing should include Free T4 (FT4) or Free T4 Index (FTI) for suspected hypothyroidism or hyperthyroidism 1
- Screening is particularly indicated for women with:
- Family or personal history of thyroid disease
- Physical findings or symptoms of goiter or hypothyroidism
- Type 1 diabetes mellitus
- Personal history of autoimmune disorders 2
Monitoring Schedule
- Measure thyroid function as soon as pregnancy is confirmed 1, 3
- Monitor at minimum during each trimester 1, 3
- For women with hypothyroidism: Check TSH and Free T4 every 4-6 weeks until stable 1
- For women with hyperthyroidism: Monitor Free T4 every 2-4 weeks 1
Management of Hypothyroidism in Pregnancy
Target TSH Levels
- Maintain serum TSH in trimester-specific reference ranges 3
- TSH should be <2.5 mIU/L in the first trimester 4
- TSH should be <3.0 mIU/L in the second trimester 5
Levothyroxine Treatment
- Women with pre-existing hypothyroidism typically need dose increases by 4-6 weeks gestation 1
- Dose increases of 30% or more may be necessary 1
- Dosing guidelines based on TSH levels:
Preconception Planning
- For women planning pregnancy, aim for TSH <1.2 mIU/L preconception to minimize the need for dose adjustments during pregnancy 7
- Women with TSH between 1.2-2.4 mIU/L preconception have a 50% chance of requiring levothyroxine dose increases during pregnancy 7
Management of Hyperthyroidism in Pregnancy
Medication Selection
- First trimester: Propylthiouracil is preferred due to lower risk of teratogenicity 1
- Second and third trimesters: Methimazole is preferred due to lower risk of hepatotoxicity 1
- Goal: Maintain Free T4 in the high-normal range using the lowest possible thioamide dosage 1
Monitoring for Side Effects
- Monitor for agranulocytosis, hepatitis, vasculitis, and thrombocytopenia with thioamide therapy 1
- Radioactive iodine is contraindicated during pregnancy and breastfeeding 1
Risks of Untreated Thyroid Disease
Maternal Risks
- Impaired cardiac and metabolic function 1
- Increased risk of preterm birth, placental abruption 1
- Heart failure (with unmanaged hyperthyroidism) 1
- Spontaneous abortion, stillbirth 1
Fetal Risks
Postpartum Considerations
Dose Adjustment
- Reduce levothyroxine dosage to pre-pregnancy levels immediately after delivery 3
- Monitor serum TSH 4-8 weeks postpartum 3
Postpartum Thyroiditis
- Affects 5-10% of women within the first year after delivery 1
- Typically follows a biphasic pattern (hyperthyroid phase followed by hypothyroid phase) 1
- Treatment:
- Beta-blockers for symptomatic relief during hyperthyroid phase
- Levothyroxine for symptomatic patients or TSH >10 mIU/L during hypothyroid phase 1
- Long-term follow-up needed as 20-40% develop permanent hypothyroidism 1
Common Pitfalls to Avoid
- Inadequate monitoring: Failure to check thyroid function regularly during pregnancy can lead to suboptimal management 1
- Insufficient dose adjustment: Many women require increased levothyroxine doses by 4-6 weeks gestation 1
- Delayed treatment: Even subclinical hypothyroidism should be treated during pregnancy to prevent adverse outcomes 2
- Overlooking postpartum changes: Failure to reduce levothyroxine dose after delivery can lead to iatrogenic hyperthyroidism 3
- Insufficient preconception planning: Women with TSH >1.2 mIU/L preconception are at higher risk of requiring dose adjustments during pregnancy 7