What is the best immunoassay for thyroid function tests?

Best Immunoassays for Thyroid Function Testing

Third-generation TSH immunoassays are the most effective first-line tests for thyroid function assessment due to their superior sensitivity (98%) and specificity (92%) compared to earlier generation assays. 1

Evolution of Thyroid Function Testing Immunoassays

TSH Assay Generations

• First-generation assays: Used single polyclonal antibody, measured many biologically inactive fragments, now obsolete 2
• Second-generation assays: "Intact PTH assays" using sandwich immunoassay technique with two antibodies
  • Sensitivity: 97.1%
  • Specificity: 97.4% 3
• Third-generation assays: "Whole PTH" or "bio-intact PTH assays" with improved antibody targeting
  • Sensitivity: 98.3%
  • Specificity: 99.2% 3
  • Functional sensitivity as low as 0.007-0.02 mIU/L 4, 5

Key Advantages of Third-Generation Assays

• Can detect extremely low TSH levels (functional sensitivity of 0.01-0.02 mU/L) 4
• Allows accurate discrimination between subclinical and overt hyperthyroidism without additional testing 4
• Permits accurate monitoring of suppressive therapy in thyroid cancer patients 4
• Can distinguish between severe non-thyroidal illness and true hyperthyroidism 4

Recommended Testing Approach

Primary Screening

• TSH testing using third-generation immunoassay as the first-line approach 1, 4
• Reference ranges:
  • Normal: 0.30-3.68 mU/L
  • Low: <0.1 mU/L
  • Elevated: >6.5 mU/L 1, 4

Diagnostic Interpretation

Condition	TSH	Free T4	Diagnosis
Subclinical Hypothyroidism	Elevated	Normal	Mild hypothyroidism
Overt Hypothyroidism	Elevated	Low	Severe hypothyroidism
Subclinical Hyperthyroidism	Low	Normal	Mild hyperthyroidism
Overt Hyperthyroidism	Low	Elevated	Severe hyperthyroidism
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Technical Considerations

Assay Methods

• Chemiluminescent immunoassay (CLIA): Provides rapid results with high sensitivity, especially useful for detecting low TSH concentrations 6
• Time-resolved immunofluorometric assay: Offers excellent functional sensitivity (0.007 mIU/L) with good precision (CV 4.1-7.7%) 5
• Automated systems: Provide benefits in sensitivity, precision, speed, and convenience 5

Common Pitfalls and Limitations

• Pre-analytical factors:

  • Serum vs. plasma measurements yield different results; laboratories should standardize sample type 2
  • Biotin supplements can interfere with assay results, causing either under or overestimation 2

• Interpretation challenges:

  • TSH may be inappropriately secreted in pituitary tumors or thyroxine resistance 7
  • Severe non-thyroidal illness can affect TSH levels, requiring careful interpretation 7
  • Heterophilic antibodies in patient serum may interfere with results 7

• Standardization issues:

  • Different manufacturers use different international standards 2
  • The International Federation of Clinical Chemistry (IFCC) is working to improve comparability between assays 2

Monitoring Recommendations

• Monitor thyroid function tests (TSH, free T4) every 6-12 months for stable patients
• Increase frequency to every 3-6 months for patients with recent dose adjustments 1

Future Directions

• Mass spectrometry-based assays show promise as reference methods due to higher specificity than immunoassays 2
• However, mass spectrometry currently has lower analytical sensitivity and requires further optimization before becoming a standard reference method 2