What is checked initially besides Thyroid-Stimulating Hormone (TSH) to diagnose hyperthyroidism in a postpartum woman?

Initial Laboratory Tests for Hyperthyroidism in Postpartum Women

In addition to TSH, Free T4 (FT4) or Free T4 Index (FTI) should be checked initially when evaluating for hyperthyroidism in a postpartum woman. 1

Diagnostic Algorithm for Postpartum Hyperthyroidism

Initial Laboratory Evaluation

• TSH (primary screening test)
• Free T4 (FT4) or Free T4 Index (FTI)
• Consider Free T3 (FT3) if clinical suspicion is high but TSH and FT4 are incongruent

Rationale for Testing Both TSH and FT4/FTI

1. TSH alone may be insufficient for diagnosis in the postpartum period due to:

   • Physiologic changes in thyroid function following pregnancy
   • Possibility of postpartum thyroiditis which has distinct phases
   • Potential for analytical interferences affecting test results

2. The American College of Obstetricians and Gynecologists (ACOG) specifically recommends that "TSH and FT4 or FTI testing should be performed in pregnant women with suspected hyperthyroidism or hypothyroidism" 1. This recommendation extends to the postpartum period, particularly for women with symptoms.

Postpartum Thyroiditis Considerations

Postpartum thyroiditis occurs in 5-10% of women and typically follows a biphasic pattern:

• Initial hyperthyroid phase (first 1-3 months postpartum)
• Later hypothyroid phase (4-8 months postpartum)
• Eventual return to normal function in most cases

The diagnosis of postpartum thyroiditis is specifically made by "new onset of an abnormal TSH level, abnormal FT4 level, or both" 1. Testing both parameters is essential for accurate diagnosis and phase identification.

Special Testing Considerations

When to Consider Additional Testing

• If clinical presentation doesn't match laboratory findings
• When TSH and FT4/FTI values are discordant
• In cases with history of autoimmune thyroid disease

Additional Tests to Consider

• Thyroid antibody testing (particularly anti-thyroid peroxidase antibodies) may be useful in confirming the diagnosis of postpartum thyroiditis 1
• Thyrotropin-releasing hormone (TRH) level in select cases 1

Clinical Pitfalls to Avoid

1. Relying solely on TSH: While TSH is the most sensitive test for thyroid dysfunction, it can be misleading in the dynamic postpartum period.

2. Ignoring clinical presentation: Laboratory values must be interpreted in the context of symptoms. Discordance between clinical status and laboratory values may indicate analytical interference, such as thyroid hormone autoantibodies 2, 3.

3. Failure to recognize analytical interferences: Circulating autoantibodies against T3 and T4 can cause falsely elevated free hormone measurements in one-step assays 3.

4. Missing the biphasic nature of postpartum thyroiditis: Testing only once may miss the transition between hyperthyroid and hypothyroid phases.

By following this systematic approach to laboratory evaluation, clinicians can accurately diagnose and appropriately manage thyroid dysfunction in postpartum women, improving maternal morbidity, mortality, and quality of life.