Normal Reference Ranges for Thyroid Function Tests in Adults
For adults without known thyroid disease, the normal reference ranges are: TSH 0.45–4.5 mIU/L, free T4 (FT4) typically 9–19 pmol/L (laboratory-specific), and free T3 (FT3) measurement is rarely needed for routine screening. 1
TSH (Thyroid-Stimulating Hormone)
- The standard reference range for TSH is 0.45–4.5 mIU/L, representing the 2.5th to 97.5th percentile in disease-free populations 1
- The geometric mean TSH in healthy individuals is approximately 1.4 mIU/L 1
- TSH demonstrates high diagnostic accuracy with >98% sensitivity and >92% specificity for detecting thyroid dysfunction 1
- TSH values exhibit substantial biological variation, with day-to-day fluctuations reaching up to 50% of the mean value and intra-day variation of approximately 40% 1
Age-Related Considerations for TSH
- In individuals aged ≥80 years, approximately 12% have TSH >4.5 mIU/L without underlying thyroid disease, indicating that the standard reference range may be inappropriately low for elderly adults 1
- The upper limit of normal TSH shifts upward with advancing age, potentially reaching 7.5 mIU/L in patients over 80 years 1
Free T4 (Free Thyroxine)
- Normal FT4 ranges are laboratory-specific but typically fall between 9–19 pmol/L (or approximately 0.7–1.5 ng/dL in conventional units) 1
- FT4 measurement is essential when TSH is abnormal to distinguish subclinical (normal FT4) from overt (abnormal FT4) thyroid dysfunction 1
- The combination of normal TSH with normal FT4 definitively excludes both overt and subclinical thyroid dysfunction 1
Free T3 (Free Triiodothyronine)
- Routine FT3 measurement is NOT recommended for initial thyroid screening in asymptomatic adults 1
- FT3 testing has limited clinical utility, with T3 thyrotoxicosis representing a rare diagnosis (approximately 0.5% of newly diagnosed hyperthyroidism cases) 2
- FT3 measurement is most useful only when TSH is suppressed (<0.01 mIU/L) and FT4 is normal or decreased, to distinguish T3 thyrotoxicosis from subclinical hyperthyroidism 2, 3
When FT3 Testing May Be Indicated
- In patients with TSH <0.01 mIU/L and normal/low FT4, particularly in the outpatient setting where T3 thyrotoxicosis frequency is higher (34% vs 14% inpatient) 2
- To evaluate for autonomous thyroid hormone production when TSH is markedly suppressed but FT4 remains normal 3
- All patients with newly diagnosed T3 thyrotoxicosis had TSH <0.01 mIU/L, making this the appropriate threshold for reflex FT3 testing 2
Important Caveats About Reference Ranges
Factors Causing Transient Abnormalities
- TSH can be transiently affected by acute illness, hospitalization, recent iodine exposure (e.g., CT contrast), certain medications, or recovery from thyroiditis 1
- These physiological factors should be considered before interpreting isolated abnormal results 1
- 30–60% of initially elevated TSH values normalize spontaneously on repeat testing, highlighting the importance of confirmation before initiating treatment 1
Biological Variation Data
- For TSH: within-subject biological variation (CVI) is 22.3%, between-subject variation (CVG) is 26.6%, and reference change value (RCV) is -40.3% to +67.6% 4
- For FT3: CVI is 4.4%, CVG is 9.2%, and RCV is -10.4% to +11.6% 4
- For FT4: CVI is 5.1%, CVG is 8.2%, and RCV is -12.7% to +14.5% 4
- Because thyroid function tests show high individuality (individuality index <0.6 for FT3 and FT4), serial monitoring using RCV is more appropriate than population-based reference ranges for individual patients 4
Clinical Interpretation Algorithm
For initial thyroid assessment:
- Measure TSH as the first-line screening test 1
- If TSH is abnormal, measure FT4 to distinguish subclinical from overt dysfunction 1
- Do NOT routinely measure FT3 unless TSH is <0.01 mIU/L with normal/low FT4 2
- Always confirm abnormal results with repeat testing after 3–6 weeks before making treatment decisions 1