Management of Newly Detected Hypothyroidism in Pregnancy
Initiate levothyroxine immediately upon diagnosis to normalize TSH and prevent adverse maternal and fetal outcomes, including preeclampsia, low birth weight, and impaired neurodevelopment. 1, 2
Immediate Treatment Initiation
Start levothyroxine without delay once hypothyroidism is confirmed in pregnancy. The urgency stems from clear evidence that untreated maternal hypothyroidism increases risks of preeclampsia, low birth weight, and neuropsychologic defects in offspring 1. The critical window for fetal brain development makes prompt treatment essential 3.
Dosing Strategy for Newly Diagnosed Hypothyroidism
The starting dose depends on TSH severity:
- TSH ≥10 IU/L (overt hypothyroidism): Start levothyroxine at 1.6 mcg/kg/day 4
- TSH <10 IU/L (subclinical hypothyroidism): Start levothyroxine at 1.0 mcg/kg/day 4
For context, a 70 kg woman with overt hypothyroidism would receive approximately 112 mcg daily, while one with mild elevation would receive 70 mcg daily 4.
Treatment Goals and Monitoring
The target is to maintain TSH below 2.5 μIU/mL, ideally within trimester-specific reference ranges. 3 More specifically:
- Aim to keep TSH in the trimester-specific reference range 2, 4
- Monitor serum TSH every 4 weeks after initiation or dose adjustment until stable 4
- Adjust levothyroxine dose by 12.5 to 25 mcg increments based on TSH results 4
- Once stable, continue monitoring at minimum during each trimester 4
The American College of Obstetricians and Gynecologists emphasizes maintaining TSH within normal trimester-specific ranges throughout pregnancy 2.
Laboratory Testing Approach
Obtain both TSH and free T4 (or Free Thyroxine Index) for initial diagnosis and monitoring. 1 TSH testing using monoclonal antibodies is the recommended initial screening test, but both TSH and FT4 should be performed when hypothyroidism is suspected 1.
Consider checking thyroid peroxidase (TPO) antibodies, as their presence increases risk of progression and may influence treatment decisions, particularly in subclinical cases 5.
Critical Clinical Pitfalls
Real-world evidence shows hypothyroidism in pregnancy is frequently overlooked or inadequately treated. 3 Common errors include:
- Failure to increase dose promptly: Many women have out-of-range TSH at pregnancy confirmation despite pre-existing treatment 5
- Inadequate monitoring frequency: TSH should be checked every 4 weeks until stable, not just once per trimester 4
- Poor medication adherence: Suboptimal adherence is common and requires effective clinician-patient communication 6
- Missing the diagnosis: Targeted screening of only high-risk women misses 30-80% of cases 5
Special Considerations
Subclinical Hypothyroidism (TSH elevated, normal FT4)
All pregnant women with TSH >10 mIU/L require treatment. 7 For TSH between 2.5-10 mIU/L, treatment is strongly recommended if:
- TPO antibodies are positive 7
- Patient is symptomatic 7
- TSH is above trimester-specific reference range 2
There is consensus that even mild subclinical hypothyroidism should be treated in pregnancy to decrease risk of complications and impaired cognitive development of offspring 7.
Risks of Undertreatment vs. Overtreatment
Undertreatment poses greater risks than overtreatment during pregnancy. 6 Inadequate treatment is associated with:
While overtreatment should be avoided, recent meta-analyses confirm that both overt and subclinical hypothyroidism are associated with adverse maternal, neonatal, and offspring outcomes 6.
Postpartum Management
Inform the newborn's physician about maternal hypothyroidism due to risk of neonatal thyroid dysfunction. 2 After delivery: