Yes, Treatment with Levothyroxine Is Indicated
A 27-week pregnant woman with TSH 7.05 mIU/L (elevated), normal FT4 1.41 ng/dL, and low FT3 2.39 pg/mL should be treated with levothyroxine immediately. This represents overt hypothyroidism in pregnancy requiring prompt intervention to prevent maternal and fetal complications. 1
Why Treatment Is Necessary
Maternal and Fetal Risks of Untreated Hypothyroidism
- Untreated maternal hypothyroidism significantly increases the risk of preeclampsia, which can be life-threatening for both mother and baby 1
- Low birth weight is strongly associated with inadequate treatment of maternal hypothyroidism 1
- Maternal hypothyroidism from any cause increases the risk of congenital cretinism in the offspring, manifesting as growth failure, mental retardation, and neuropsychological defects 1
- Additional risks include spontaneous abortion, gestational hypertension, stillbirth, and premature delivery 2
- Untreated maternal hypothyroidism may have adverse effects on fetal neurocognitive development 2
TSH Threshold for Treatment
- The American Academy of Family Physicians recommends treating all pregnant women with elevated TSH using levothyroxine to restore TSH to trimester-specific reference ranges 1
- Your patient's TSH of 7.05 mIU/L exceeds the upper limit for the second trimester (normal range approximately 0.51-4.05 mIU/L based on gestation-specific data) 3, 4
- Even though FT4 appears "normal," pregnancy-specific reference ranges differ from non-pregnant values, and elevated TSH alone warrants treatment 1
Treatment Protocol
Initiation of Levothyroxine
- Start levothyroxine sodium immediately upon diagnosis; do not delay treatment 2
- Levothyroxine should not be discontinued during pregnancy 2
- The FDA label explicitly states that hypothyroidism diagnosed during pregnancy should be promptly treated 2
Monitoring Strategy
- Serum TSH levels should be monitored regularly during pregnancy, as TSH may increase as pregnancy progresses 2
- Levothyroxine dosage must be adjusted during pregnancy to maintain TSH within trimester-specific reference ranges 1, 2
- Pregnancy may increase levothyroxine requirements, necessitating dose escalation 2
Postpartum Management
- After delivery, the levothyroxine dose should return to the pre-pregnancy dose immediately, as postpartum TSH levels typically revert to preconception values 2
Important Clinical Pitfalls to Avoid
Do Not Use Non-Pregnant Reference Ranges
- Thyroid hormone reference ranges in pregnant women differ substantially from non-pregnant women 5, 3
- TSH tends to increase progressively through the three trimesters, while FT4 and FT3 typically decline 3, 6
- Using non-pregnant reference intervals can lead to misclassification of thyroid status 4
Do Not Dismiss Low FT3 as Insignificant
- While the low FT3 (2.39 pg/mL) in your patient may reflect the normal physiologic decline seen in later pregnancy 3, 6, the elevated TSH is the critical diagnostic finding that mandates treatment
- The combination of elevated TSH with "normal" FT4 still represents hypothyroidism requiring intervention 1
Adequate Treatment Is Essential
- Inadequate treatment—not just absence of treatment—is associated with low birth weight in neonates 1
- The goal is to normalize TSH to trimester-specific ranges, not simply to initiate therapy 1
Breastfeeding Considerations
- Levothyroxine is present in human milk, but no adverse effects on breastfed infants have been reported 2
- Adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid lactating mothers with low milk supply 2
- The developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for levothyroxine 2