What are the initial labs for diagnosing Hashimoto's (Hashimoto's thyroiditis)?

Initial Laboratory Tests for Diagnosing Hashimoto's Thyroiditis

The initial laboratory evaluation for diagnosing Hashimoto's thyroiditis should include thyroid stimulating hormone (TSH), free thyroxine (free T4), and thyroid antibodies, specifically thyroid peroxidase antibodies (TPO-Ab) and thyroglobulin antibodies (TG-Ab).

Core Diagnostic Tests

Primary Tests

• Thyroid Stimulating Hormone (TSH): The most sensitive initial screening test for thyroid dysfunction 1
• Free Thyroxine (Free T4): To distinguish between subclinical and overt hypothyroidism 1
• Thyroid Antibodies:
  • Thyroid Peroxidase Antibodies (TPO-Ab): Present in 90-95% of Hashimoto's patients
  • Thyroglobulin Antibodies (TG-Ab): Present in approximately 60-80% of patients 2, 3

Interpretation of Results

• Typical Hashimoto's pattern: Elevated TSH, normal or low free T4, positive thyroid antibodies (TPO-Ab and/or TG-Ab)
• Early/Subclinical Hashimoto's: Normal TSH, normal free T4, positive thyroid antibodies
• Advanced Hashimoto's: Markedly elevated TSH, low free T4, positive thyroid antibodies

Clinical Significance of Antibody Testing

Recent evidence shows that thyroid antibodies correlate with symptom burden in Hashimoto's thyroiditis, even in patients with normal thyroid hormone levels:

• TG-Ab levels show significant positive correlation with number of symptoms (r = 0.25, p = 0.0001) 2
• Elevated TG-Ab levels are specifically associated with fragile hair, facial edema, eye edema, and harsh voice 2
• Both TPO-Ab and TG-Ab are positively correlated with inflammatory markers (TNF-α and IFN-γ) and symptoms including abdominal distension, diarrhea, chilliness, forgetfulness, and fatigue 4

Additional Considerations

Follow-up Testing

• If initial TSH is abnormal, repeat testing should be performed within 3-6 months to confirm the diagnosis 1
• Multiple TSH measurements over time provide more reliable diagnostic information than a single test

Imaging Studies

While not part of initial laboratory testing, these may be considered after lab results:

• Thyroid Ultrasound: To evaluate thyroid structure and confirm the typical hypoechogenic pattern characteristic of Hashimoto's thyroiditis 5

Common Pitfalls to Avoid

1. Relying solely on TSH: Some patients may have normal TSH but positive antibodies in early disease
2. Testing only TPO-Ab: While TPO-Ab is more common, some patients are only TG-Ab positive
3. Ignoring antibody levels in treated patients: Antibody levels may remain elevated despite treatment with levothyroxine 5
4. Misinterpreting transient thyroid dysfunction: Hashimoto's can follow Graves' disease or present with periods of hyperthyroidism 6

Monitoring Recommendations

For patients diagnosed with Hashimoto's thyroiditis:

• TSH and free T4 should be monitored every 6-12 months
• Antibody levels typically decline during levothyroxine treatment but become negative in only about 16% of patients after 50 months 5

Remember that Hashimoto's thyroiditis is the most common cause of hypothyroidism in the United States 1, and proper laboratory diagnosis is essential for appropriate management and to improve quality of life.