What is the breakdown and interpretation of thyroid labs, including TSH (Thyroid-Stimulating Hormone), FT4 (Free Thyroxine), and FT3 (Free Triiodothyronine)?

Thyroid Labs Breakdown and Interpretation

The most effective approach to interpreting thyroid labs requires understanding the relationship between TSH, FT4, and FT3, with TSH being the primary screening test and FT4/FT3 providing additional diagnostic information in specific clinical scenarios. 1

Normal Reference Ranges

• TSH (Thyroid-Stimulating Hormone): 0.45-4.5 mIU/L
• FT4 (Free Thyroxine): Varies by laboratory, typically 0.8-1.8 ng/dL
• FT3 (Free Triiodothyronine): Varies by laboratory, typically 2.3-4.2 pg/mL

Primary Diagnostic Approach

TSH as First-Line Test

• TSH is the most sensitive indicator of thyroid function and should be the initial screening test
• TSH has an inverse relationship with thyroid hormone levels
• Abnormal TSH should prompt further testing with FT4 and sometimes FT3

When to Add FT4 Testing

• Always measure when TSH is abnormal
• Helps distinguish between subclinical and overt thyroid dysfunction
• Essential for diagnosing central hypothyroidism (low/normal TSH with low FT4)

When to Add FT3 Testing

• Most useful when TSH is suppressed (<0.1 mIU/L) and FT4 is normal or low 2
• Helps identify T3 thyrotoxicosis
• Limited utility in most other clinical scenarios
• Consider when clinical hyperthyroidism is suspected despite normal FT4

Interpretation of Common Patterns

Primary Hypothyroidism

• Overt: Elevated TSH (>4.5 mIU/L) with low FT4
• Subclinical: Elevated TSH with normal FT4
• Action: TSH >10 mIU/L with symptoms warrants treatment 1

Primary Hyperthyroidism

• Overt: Suppressed TSH (<0.45 mIU/L) with elevated FT4 and/or FT3
• Subclinical: Suppressed TSH with normal FT4 and FT3
• T3 Thyrotoxicosis: Suppressed TSH (<0.01 mIU/L) with normal FT4 but elevated FT3 2

Central Hypothyroidism

• Low/normal TSH with low FT4
• Requires evaluation for pituitary or hypothalamic dysfunction
• Rare finding (approximately 2 cases/100,000 population/year) 3

Special Considerations

Subclinical Hyperthyroidism Management

For TSH 0.1-0.45 mIU/L:

• Repeat TSH, measure FT4 and FT3 for confirmation
• If persistent, monitor every 3-12 months
• Treatment generally not recommended unless elderly or cardiac disease present 1

For TSH <0.1 mIU/L:

• Repeat TSH, FT4, and FT3 within 4 weeks
• Consider treatment for persistent findings, especially with symptoms or risk factors
• Evaluate for etiology (radioactive iodine uptake scan helpful) 1

Pregnancy Considerations

• TSH reference ranges are trimester-specific
• TSH should be measured each trimester in treated hypothyroid women
• FT4 helps guide therapy when TSH is abnormal 1

Medication Effects

• Levothyroxine treatment requires monitoring TSH to guide dosing
• Goal TSH depends on indication:
  • Primary hypothyroidism: TSH within reference range
  • Thyroid cancer: May target lower TSH (0.1-0.5 mIU/L) 1, 4
• Monitor TSH 6-8 weeks after dose adjustments 1

Common Pitfalls

1. Ignoring assay variability: Different laboratory methods yield significantly different FT3 and FT4 results 5, 6

2. Overreliance on FT3: FT3 has limited utility in most clinical scenarios and should be reserved for specific indications 2

3. Missing central hypothyroidism: Low/normal TSH with low FT4 requires further evaluation 3

4. Non-thyroidal illness: Acute illness can cause abnormal thyroid tests without true thyroid dysfunction

5. Medication interference: Many drugs affect thyroid hormone levels and binding proteins 4

6. Heterophilic antibodies: Can cause falsely elevated FT4 and FT3 with normal TSH 5

7. Failure to confirm abnormal results: Single abnormal test should be confirmed before initiating treatment 1

By following this systematic approach to thyroid function testing, clinicians can accurately diagnose and monitor thyroid disorders while avoiding unnecessary testing and treatment.