Thyroid Function Testing: Clinical Protocol and Interpretation Guide

Pre-analytical Considerations

Patient Preparation

• Fast for 8-12 hours (if lipid panel included)
• Morning collection preferred (7-9 AM)
• Medication timing considerations:
  • Hold biotin supplements for 48 hours
  • Document timing of levothyroxine dose
  • Note other interfering medications

Required Materials

• SST or plain tubes for serum
• Appropriate labels and requisition forms
• Temperature-controlled storage

Core Testing Panel

Primary Tests

1. Thyroid Stimulating Hormone (TSH)

   • Reference range: 0.4-4.0 mIU/L
   • First-line screening test

2. Free Thyroxine (FT4)

   • Reference range: 0.8-1.8 ng/dL
   • Essential for diagnosis confirmation

Secondary Tests (as indicated)

• Free Triiodothyronine (FT3)
• Thyroid antibodies (TPOAb, TgAb)
• Thyroid-stimulating immunoglobulins (TSI)

Interpretation Guidelines

Common Patterns

1. Primary Hypothyroidism

   • ↑ TSH, ↓ FT4

2. Primary Hyperthyroidism

   • ↓ TSH, ↑ FT4

3. Subclinical Disease

   • Hypothyroid: ↑ TSH, normal FT4
   • Hyperthyroid: ↓ TSH, normal FT4

Follow-up Protocol

Timing of Repeat Testing

• New levothyroxine therapy: 6-8 weeks
• Dose adjustments: 4-6 weeks
• Stable patients: 6-12 months

Documentation Requirements

• Record all results in EMR
• Document clinical correlation
• Include medication adjustments
• Note follow-up plan

Special Considerations

Pregnancy

• Trimester-specific TSH ranges
• More frequent monitoring
• Modified treatment targets

Critical Values

• TSH >50 mIU/L
• TSH <0.01 mIU/L with elevated FT4
• Immediate physician notification required