ACOG Guidelines for Initiating Levothyroxine in Pregnancy
Pregnant women with elevated TSH should be treated with levothyroxine immediately to restore TSH to the reference range, and women already on levothyroxine should increase their dose by 30% as soon as pregnancy is confirmed. 1
Screening and Case Finding
TSH testing should be obtained in pregnant women with specific risk factors, not universally: 1
- Family or personal history of thyroid disease
- Physical findings or symptoms of goiter or hypothyroidism
- Type 1 diabetes mellitus
- Personal history of autoimmune disorders
- History of thyroid nodules or goiter 2
Treatment Initiation for Newly Diagnosed Hypothyroidism
Overt Hypothyroidism (TSH >10 mIU/L)
Start levothyroxine at 1.6 mcg/kg/day immediately. 3
Subclinical Hypothyroidism (Elevated TSH with Normal Free T4)
Treat with levothyroxine to restore TSH to reference range based on the potential association between elevated TSH and increased fetal wastage or neuropsychological complications in offspring. 1
The rationale for treatment is compelling despite lack of intervention trials, as the benefit-risk ratio of levothyroxine therapy justifies its use given minimal risks with appropriate management. 1
Dose Adjustment for Women Already on Levothyroxine
Increase levothyroxine dose by 12.5 to 25 mcg per day (approximately 30% increase) immediately upon pregnancy confirmation. 3, 4
This recommendation is critical because:
- Levothyroxine requirements increase as early as the fifth week of gestation 4
- The mean requirement increases 47% during the first half of pregnancy, with median onset at 8 weeks gestation 4
- The increased dose plateaus by week 16 and is required until delivery 4
Monitoring Strategy
Monitor TSH every 4 weeks until stable, then continue monitoring throughout pregnancy: 3
- Check TSH and free T4 as soon as pregnancy is confirmed 3
- Monitor at minimum during each trimester 3
- Adjust dose every 6 to 8 weeks during pregnancy as needed 1
Target TSH Levels
Maintain TSH in trimester-specific reference ranges: 3
- First trimester: TSH ≤2.5 mIU/L
- Second and third trimesters: TSH ≤3.0 mIU/L
Postpartum Management
Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery and monitor TSH 4 to 8 weeks postpartum. 3
Critical Clinical Pitfalls
The magnitude of dose increase varies by etiology of hypothyroidism: 5
- Primary hypothyroidism requires smaller increases (16% cumulative by second trimester)
- Treated Graves' disease or goiter requires the largest increases (51% cumulative by second trimester)
- Thyroid cancer patients require intermediate increases (21-26% cumulative)
Failure to increase the dose early enough is a common error that can result in biochemical hypothyroidism occurring in many pregnant women despite being on treatment. 4