Normal Thyroid Profile Values
The normal reference range for thyroid function tests includes TSH between 0.45-4.12 mU/L, with most healthy individuals having TSH levels below 2.5 mU/L. 1, 2
Standard Reference Ranges
Thyroid-Stimulating Hormone (TSH)
- Normal range: 0.45-4.12 mU/L 1
- Most truly normal individuals have TSH below 2.5 mU/L 2
- The true normal mean TSH in populations with low incidence of thyroid disease is approximately 1.18 mU/L 2
Free Thyroxine (FT4)
- Normal range: 11-20 pmol/L (approximately 0.8-1.8 ng/dL)
- Median FT4 in euthyroid patients on levothyroxine: 16 pmol/L 3
- 20-80th centile range in treated patients: 14-19 pmol/L 3
Free Triiodothyronine (FT3)
- Normal FT3/FT4 ratio in euthyroid individuals: approximately 0.32 4
- FT3 values must be interpreted in relation to TSH and FT4
Clinical Interpretation of Thyroid Function Tests
Patterns of Thyroid Dysfunction
- Elevated TSH with normal FT4: Subclinical hypothyroidism 1
- Elevated TSH with low FT4: Overt hypothyroidism 1
- Low TSH with normal FT4: Subclinical hyperthyroidism 1
- Low TSH with elevated FT4: Overt hyperthyroidism 1
Age-Related Considerations
- FT3/FT4 ratio varies with age:
Important Clinical Caveats
- Laboratory reference ranges may vary between different assay methods and laboratories
- The historical reference ranges for TSH may be skewed by inclusion of individuals with subclinical thyroid disease 2
- A falling TSH across two measurements with normal/low T4 may suggest pituitary dysfunction rather than primary thyroid disease 1
- When interpreting thyroid function tests in patients with suspected pituitary disease, TSH values may be misleadingly normal despite thyroid dysfunction 3
Monitoring Considerations
- For patients on thyroid hormone replacement therapy, target TSH should be in the normal range (0.45-4.12 mU/L) 1
- For reproductive-age women, a more narrow TSH target of 1.0-2.5 mIU/L is recommended 1
- Regular monitoring of thyroid function is recommended every 4-6 weeks during dose adjustments and every 3-6 months once stable 1
Understanding these normal ranges and their clinical interpretation is essential for accurate diagnosis and management of thyroid disorders, with recognition that age, assay methods, and individual factors may influence the interpretation of results.