Thyroid Function Testing After Pregnancy
In addition to serum TSH, free T4 (FT4) or Free T4 Index (FTI) should be measured when evaluating thyroid function after pregnancy, along with thyroid antibody testing to confirm diagnosis of postpartum thyroiditis. 1
Comprehensive Postpartum Thyroid Evaluation
Essential Laboratory Tests
- TSH: Basic screening test but insufficient alone in postpartum period
- Free T4 (FT4) or Free T4 Index (FTI): Critical for accurate diagnosis of thyroid dysfunction phases
- Thyroid antibodies: Particularly anti-thyroid peroxidase antibodies to confirm postpartum thyroiditis
- Free T3 (FT3): Consider when clinical suspicion is high but TSH and FT4 are incongruent
Timing of Testing
- Initial evaluation should include both TSH and FT4/FTI
- For women with identified thyroid dysfunction, follow-up testing should occur at appropriate intervals:
Postpartum Thyroiditis Considerations
Postpartum thyroiditis affects 5-10% of women and typically follows a biphasic pattern 1:
- Initial hyperthyroid phase
- Later hypothyroid phase
- Most women return to normal function eventually
Testing both TSH and FT4/FTI is essential because:
- The biphasic nature of postpartum thyroiditis requires monitoring of both parameters
- Relying on TSH alone may miss the transition between hyperthyroid and hypothyroid phases
- Physiologic changes in thyroid function following pregnancy can affect test interpretation
Clinical Application
When to Test
- Women who develop a goiter during pregnancy or after delivery
- Women with postpartum symptoms of hyperthyroidism or hypothyroidism
- Women with a personal or family history of thyroid disease
- Women with type 1 diabetes or autoimmune disorders 2
Interpretation Challenges
- Laboratory values must be interpreted in context of symptoms
- Discordance between clinical status and laboratory values may indicate analytical interference
- The risk of permanent hypothyroidism is greatest in women with the highest levels of TSH and antithyroid peroxidase antibodies 2
Treatment Monitoring
For women diagnosed with hypothyroidism requiring levothyroxine:
- Monitor serum TSH every 4-8 weeks postpartum 3
- Adjust dosage to maintain TSH within the reference range
- Consider reducing levothyroxine dosage to pre-pregnancy levels immediately after delivery if treatment was started or increased during pregnancy 3
By including FT4/FTI and thyroid antibody testing along with TSH, clinicians can more accurately diagnose and monitor thyroid dysfunction in the postpartum period, leading to better management of this common condition.