What labs are included in a full thyroid workup?

Comprehensive Thyroid Workup: Essential Laboratory Tests

A complete thyroid workup should include TSH, Free T4, Free T3, thyroid antibodies (TPO-Ab and TRAb), and thyroglobulin with thyroglobulin antibodies, with additional specialized tests based on clinical suspicion.

Core Thyroid Laboratory Tests

First-Line Tests

• Thyroid Stimulating Hormone (TSH): The most sensitive initial screening test for thyroid dysfunction 1, 2
• Free Thyroxine (FT4): Essential for confirming hypo/hyperthyroidism and assessing severity 3
• Free Triiodothyronine (FT3): Important when TSH is suppressed but FT4 is normal to identify T3 toxicosis 3, 4

Second-Line Tests

• Thyroid Antibodies:
  • Thyroid Peroxidase Antibodies (TPO-Ab): For diagnosing Hashimoto's thyroiditis 5
  • TSH Receptor Antibodies (TRAb): For confirming Graves' disease 5
  • Thyroglobulin Antibodies (Tg-Ab): Important for interpreting thyroglobulin results 5

Specialized Tests

• Thyroglobulin (Tg): Tumor marker for differentiated thyroid carcinoma follow-up 5
• Calcitonin: For screening and monitoring medullary thyroid carcinoma 1, 5

Testing Algorithm Based on Clinical Scenario

For Initial Thyroid Evaluation

1. Start with TSH measurement 1, 2
2. If TSH is abnormal, add FT4 to further characterize the abnormality 6
3. If TSH is suppressed but FT4 is normal, add FT3 to rule out T3 toxicosis 3, 4

For Suspected Autoimmune Thyroid Disease

1. Measure TSH and FT4
2. Add TPO-Ab for suspected Hashimoto's thyroiditis 5
3. Add TRAb for suspected Graves' disease 5

For Thyroid Cancer Evaluation

1. Include TSH, FT4, and FT3 for functional assessment
2. Add thyroglobulin and thyroglobulin antibodies for differentiated thyroid cancer 1, 5
3. Add calcitonin for suspected medullary thyroid carcinoma 1, 5

Special Considerations

Pregnancy

• TSH reference ranges differ during pregnancy
• FT4 measurement is more reliable than total T4 during pregnancy 3

Critical Illness

• Thyroid function tests may be altered in non-thyroidal illness
• Interpretation should consider clinical context 5

Medication Interference

• Many medications can affect thyroid function test results
• Document all current medications when ordering tests 5

Potential Pitfalls in Thyroid Testing

Laboratory Interferences

• Heterophilic antibodies can cause falsely elevated TSH in immunometric assays 7
• Thyroid hormone autoantibodies may interfere with FT4 and FT3 measurements 7
• Abnormal albumin variants can affect analog-based free hormone assays 7

Clinical Interpretation Challenges

• Discordant results between TSH and free hormone levels require careful evaluation 7
• Subclinical thyroid dysfunction (abnormal TSH with normal free hormone levels) needs proper interpretation 4

Cost-Effective Approach

• Starting with TSH alone is appropriate for initial screening 4, 6
• Adding FT4 when TSH is abnormal is the most cost-effective approach 4
• Ordering panels that include both T3 and FT3 is generally not justified 4

By following this structured approach to thyroid testing, clinicians can efficiently diagnose and monitor thyroid disorders while avoiding unnecessary tests and potential interpretation errors.