TSH Goals for Pregnant Women with Hyperthyroidism
For pregnant women with hyperthyroidism, the target TSH level should be maintained in the low-normal range, with treatment goals of ≤2.5 mIU/L for the first trimester and ≤3.0 mIU/L for the second and third trimesters. 1
Understanding Thyroid Function in Pregnancy
- Pregnancy causes significant changes in thyroid physiology, including increased thyroid binding globulin synthesis and thyroid stimulation from human chorionic gonadotropin (hCG) 2
- Free T4 levels typically decrease during the latter half of pregnancy 2
- Proper thyroid hormone levels are crucial for fetal development, particularly brain development 3
Recommended TSH Reference Ranges by Trimester
- First trimester: 0.1 to 2.5 mIU/L 1
- Second trimester: 0.2 to 3.0 mIU/L 1
- Third trimester: 0.3 to 3.0 mIU/L 1
Management of Hyperthyroidism in Pregnancy
- The medical management of hyperthyroidism during pregnancy consists of monotherapy with antithyroid drugs (ATDs) 2
- Propylthiouracil (PTU) is the preferred medication during the first trimester due to lower risk of teratogenicity compared to methimazole 4
- Medication dosage should be adjusted based on free T4 in the high-normal range and TSH in the low-normal range to minimize risk of fetal hypothyroidism 2
- Doses can often be reduced in the third trimester due to the immune-suppressant effects of pregnancy 1
Monitoring Protocol
- TSH levels should be monitored every 4 weeks until stable 5
- Once stable, TSH should be checked every trimester 5
- Target is to maintain free T4 in the upper normal range while keeping TSH at appropriate trimester-specific levels 1
Risks of Untreated or Inadequately Treated Hyperthyroidism
- Pregnancy complications including preeclampsia 5
- Low birth weight in neonates 5
- Fetal distress 1
- Potential neuropsychological complications in offspring 5
- Increased fetal wastage 5
Special Considerations
- Transplacental passage of maternal TSH receptor stimulating antibodies may cause fetal hyperthyroidism, requiring careful monitoring 2
- Gestational transient thyrotoxicosis, associated with high hCG levels in the first trimester, is another cause of maternal hyperthyroidism that should be distinguished from Graves' disease 2, 3
- Women with hyperthyroidism planning pregnancy should consider definitive therapy or appropriate medical management prior to conception 2
Common Pitfalls to Avoid
- Failing to distinguish between Graves' disease and gestational transient thyrotoxicosis, as treatment approaches differ 3
- Inadequate monitoring of thyroid function throughout pregnancy 5
- Excessive ATD dosing that could lead to fetal hypothyroidism 2
- Undertreatment that fails to control maternal hyperthyroidism, leading to adverse pregnancy outcomes 3