What are the recommended TSH (Thyroid-Stimulating Hormone) and free thyroxine (FT4) levels in pregnancy?

Recommended TSH and Free Thyroxine Levels in Pregnancy

For pregnant women, TSH should be maintained in the trimester-specific reference range, with a general target of less than 2.5 mIU/L in the first trimester and less than 3.0 mIU/L in the second and third trimesters, while Free T4 should be maintained in the upper half of the normal range. 1

Thyroid Function Changes During Pregnancy

Pregnancy induces significant physiological changes in thyroid function:

• Human chorionic gonadotropin (hCG) stimulates the maternal thyroid gland, increasing thyroid hormone production 2
• Maternal thyroid hormones are essential for fetal development, especially during the first trimester when the fetal thyroid is not yet functional 2
• These changes necessitate different reference ranges for thyroid function tests during pregnancy compared to non-pregnant states

Recommended Reference Ranges

TSH Levels

• First trimester: <2.5 mIU/L 1, 2
• Second trimester: <3.0 mIU/L 1, 3
• Third trimester: <3.0 mIU/L 1

Free T4 Levels

• Should be maintained in the upper half of the normal range throughout pregnancy 1, 4
• This is particularly important for women with secondary or tertiary hypothyroidism 4

Monitoring Recommendations

The American Thyroid Association recommends 1:

• TSH and Free T4 measurements as soon as pregnancy is confirmed
• Continued monitoring at minimum during each trimester
• More frequent monitoring (every 4-6 weeks) until TSH levels stabilize

For women with pre-existing hypothyroidism 4:

• Measure serum TSH and free-T4 as soon as pregnancy is confirmed
• Monitor at minimum during each trimester
• Adjust levothyroxine dosage to maintain TSH in the trimester-specific reference range

Clinical Implications

Importance of Optimal Thyroid Function

• Untreated thyroid disease can lead to impaired maternal cardiac and metabolic function 1
• Reduces oxygen and nutrient delivery to the fetus 1
• Increases risks of preterm birth, low birth weight, placental abruption, and fetal death 1
• May cause cognitive impairment in children 1

Treatment Considerations for Hypothyroidism

For women with pre-existing hypothyroidism:

• Levothyroxine dose typically needs to increase by 4-6 weeks gestation, possibly by 30% or more 1
• The FDA-approved levothyroxine dosing guidelines recommend 4:
  • For new onset hypothyroidism with TSH ≥10 IU/L: 1.6 mcg/kg/day
  • For new onset hypothyroidism with TSH <10 IU/L: 1.0 mcg/kg/day

Pitfalls and Caveats

1. Inadequate pre-conception TSH control:

   • Women with pre-conception TSH >1.2 mIU/L have a 50% chance of requiring levothyroxine dose increases during pregnancy 5
   • When pre-conception TSH is <1.2 mIU/L, only 17.2% require dose increases 5

2. Suboptimal monitoring:

   • Studies show that 43% of hypothyroid pregnant women have TSH values above recommended guidelines in the first trimester 3
   • Alternative approaches to define reference intervals result in considerable overdiagnosis and underdiagnosis compared to population- and trimester-specific reference intervals 6

3. Dosage adjustment challenges:

   • Even with appropriate initial dosing based on baseline TSH levels, 10-20% of women with subclinical hypothyroidism require dose adjustments during the second and third trimesters 7

Screening Recommendations

The American Medical Association recommends screening for thyroid dysfunction in pregnant women with 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

Treatment Algorithm

1. Initial assessment:

   • Measure TSH and Free T4 as soon as pregnancy is confirmed
   • Use TSH as the initial screening test for thyroid dysfunction

2. Interpretation:

   • Compare results to trimester-specific reference ranges
   • Consider treatment if TSH is elevated above trimester-specific cutoffs
   • Evaluate Free T4 to distinguish between subclinical and overt hypothyroidism

3. Treatment initiation:

   • For pre-existing hypothyroidism: Increase pre-pregnancy dose by 12.5-25 mcg/day
   • For new-onset hypothyroidism: Start levothyroxine at 1.0-1.6 mcg/kg/day based on TSH level

4. Monitoring:

   • Check TSH and Free T4 every 4-6 weeks until stable
   • Continue monitoring at least once per trimester thereafter
   • Adjust dosage to maintain TSH in trimester-specific range and Free T4 in upper half of normal range

5. Postpartum adjustment:

   • Reduce levothyroxine dose to pre-pregnancy levels immediately after delivery
   • Monitor TSH 4-8 weeks postpartum