What labs are recommended for testing for autoimmune thyroiditis?

Laboratory Testing for Autoimmune Thyroiditis

The recommended laboratory tests for diagnosing autoimmune thyroiditis include TSH, free T4, and anti-thyroid peroxidase (TPO) antibodies as the primary diagnostic panel, with anti-thyroglobulin (Tg) antibodies as a supplementary test in specific cases.

Primary Diagnostic Tests

Essential Tests

• TSH and Free T4: These are the cornerstone tests for evaluating thyroid function 1
  • TSH is the recommended initial screening test using monoclonal antibody assays 1
  • Free T4 should be measured alongside TSH to determine thyroid status

Antibody Testing

• Anti-thyroid peroxidase (TPO) antibodies:

  • Should be the primary antibody test for autoimmune thyroiditis 2
  • Most predictive of thyroid dysfunction in autoimmune conditions 1
  • Use third-generation ultrasensitive methods for detection 2

• Anti-thyroglobulin (Tg) antibodies:

  • Should be measured in patients with:
    • Suspected autoimmune thyroiditis who test negative for anti-TPO antibodies 2
    • Symptom burden despite normal thyroid function tests 3
    • When thyroglobulin measurement is planned (as antibodies may interfere with testing) 2

Additional Testing Based on Clinical Presentation

For Thyrotoxic Phase

If the patient presents with symptoms of thyrotoxicosis (weight loss, palpitations, heat intolerance):

• T3 or Free T3: Helpful in highly symptomatic patients with minimal free T4 elevations 1
• TSH receptor antibodies (TRAb): Consider if clinical features suggest Graves' disease 1
• Imaging studies: Consider radioactive iodine uptake scan (RAIUS) or Technetium-99m thyroid scan to differentiate thyroiditis from Graves' disease 1

For Monitoring and Special Situations

• Repeat TSH and Free T4: Every 2-3 weeks during thyrotoxic phase of thyroiditis to detect transition to hypothyroidism 1
• TSH and Free T4: Every 1-2 years for monitoring in patients with positive antibodies 1, 4

Clinical Pearls and Pitfalls

• Thyroid function tests may be misleading if performed during acute illness or at diagnosis of other conditions like diabetes (euthyroid sick syndrome) 1
• Women with both elevated TSH and positive thyroid antibodies have approximately 5% annual risk of developing overt hypothyroidism 4
• Free T4 is the most sensitive parameter for detecting early thyroid dysfunction in subclinical hypothyroidism 5
• Anti-TPO antibodies are more predictive than anti-thyroglobulin antibodies for thyroid dysfunction 1
• Anti-thyroglobulin antibodies correlate with symptom burden (fragile hair, facial edema, eye edema, harsh voice) even in patients with normal thyroid function 3

Testing Algorithm

1. Initial evaluation: TSH, Free T4, and anti-TPO antibodies
2. If TSH normal but symptoms present: Add anti-Tg antibodies
3. If thyrotoxic symptoms present: Add T3/Free T3 and consider TRAb
4. If both TSH and antibodies abnormal: Begin close monitoring (every 2-3 months)
5. If initial tests normal but high clinical suspicion: Consider repeating tests in 3-6 months

This systematic approach to laboratory testing allows for accurate diagnosis of autoimmune thyroiditis and appropriate monitoring of disease progression.