What are the best immunoassays for free Thyroxine (fT4) and free Triiodothyronine (fT3)?

Best Immunoassays for Free T4 and Free T3 Measurement

Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) is the most accurate method for measuring free T4 (fT4) and free T3 (fT3) due to its superior specificity and sensitivity compared to conventional immunoassays. 1

Comparison of Available Methods

LC-MS/MS (Gold Standard)

• Provides the highest specificity through combination of chromatography with detection based on mass/charge ratio
• Significantly more specific than immunoassays
• Considered the reference measurement method by the International Federation of Clinical Chemistry (IFCC)
• Advantages:
  • Eliminates cross-reactivity issues common in immunoassays
  • Can discriminate between different hormone fragments
  • Superior analytical precision

Immunoassay Options (When LC-MS/MS is Unavailable)

Third-Generation Immunoassays

• Preferred when LC-MS/MS is not available
• Use sandwich-type immunoassay technology with antibodies directed against specific amino acid regions
• More specific than older generation assays
• Examples include:
  • Electrochemiluminescence immunoassay (ECLIA) - particularly useful in patients with autoantibodies to thyroid hormones 2
  • Chemiluminescent immunoassay (CLIA) - provides more detailed and reliable results 3

Equilibrium Dialysis/RIA

• Considered the reference method among immunoassay techniques for free hormone measurements 4
• More labor-intensive but provides better accuracy than direct immunoassays

Clinical Application Guidelines

For Routine Clinical Use

1. When available, use LC-MS/MS for the most accurate measurement
2. For monitoring central hypothyroidism:
   • Target free T4 in the upper half of the normal range 5
   • Do not rely on TSH for monitoring in central hypothyroidism 5

For Thyroid Disease Diagnosis

• Measure both TSH and free T4 simultaneously for accurate diagnosis 5
• When evaluating hyperthyroidism with normal fT4, consider adding fT3 measurement 5
• For monitoring treatment in central hypothyroidism, focus on free T4 rather than TSH 5

Important Considerations and Pitfalls

Method Variability

• Significant inter-assay variations exist between different immunoassay platforms 6
• Reference ranges are method-specific and not interchangeable between different assays
• When switching methods, be aware that results may differ significantly

Interference Issues

• Immunoassays can be affected by:
  • Anti-thyroid hormone antibodies
  • Abnormal binding proteins
  • Severe non-thyroidal illness
  • Certain medications

Special Situations

• In patients with autoantibodies to T3, ECLIA measurement is more accurate than RIA 2
• In severe non-thyroidal illness, all immunoassays may be less reliable, making LC-MS/MS the preferred option

Recommended Testing Protocol

1. First choice: LC-MS/MS for both fT4 and fT3 when available
2. Second choice: Third-generation immunoassays (ECLIA or CLIA)
3. Always interpret results in the context of the specific assay's reference range
4. Consider method limitations when evaluating patients with complex thyroid disorders or severe illness

By selecting the appropriate assay method and understanding its limitations, clinicians can optimize the accuracy of thyroid function assessment and improve patient outcomes.