Normal Values for Thyroid-Stimulating Hormone (TSH)
The reference range for normal TSH levels is 0.45 to 4.5 mIU/L for the general adult population. 1
Understanding TSH Reference Ranges
TSH is the most sensitive indicator of thyroid function, with its reference range established based on population studies. According to high-quality evidence from expert panels:
- The National Health and Nutrition Examination Survey (NHANES III) established a reference range of 0.45 to 4.12 mIU/L (2.5th-97.5th percentile) in a disease-free population 1
- For clinical practice purposes, this has been standardized to 0.45 to 4.5 mIU/L 1
Age and Population Considerations
TSH values can vary slightly based on demographic factors:
- Age: TSH levels tend to increase with age, particularly in those over 65 years 1
- Race: African-Americans have lower TSH levels (approximately one-third the prevalence of elevated TSH compared to whites) 1
- Sex: Women tend to have slightly higher median TSH levels than men 2
Clinical Interpretation of TSH Values
Subclinical Hypothyroidism Classification
- Mild elevation: TSH 4.5-10.0 mIU/L with normal free T4 1, 3
- More severe elevation: TSH >10.0 mIU/L with normal free T4 1, 3
Subclinical Hyperthyroidism Classification
Important Clinical Considerations
Controversy About Upper Limit
Some researchers have proposed lowering the upper reference limit:
- Some suggest 2.5 mIU/L as the upper limit 4
- Others propose approximately 3.4 mIU/L 5
- However, the most widely accepted clinical upper limit remains 4.5 mIU/L 1
Pitfalls in TSH Interpretation
- Single measurement limitations: A single abnormal TSH value should not be used for diagnosis or treatment decisions 1
- Confirmation needed: Abnormal TSH should be confirmed with repeat testing after 2-3 months 3
- Variability factors: TSH can be affected by:
- Acute illness
- Certain medications (dopamine, glucocorticoids, dobutamine)
- Pregnancy
- Recovery from thyroiditis
- Time of day (diurnal variation)
Treatment Thresholds
- TSH >10.0 mIU/L is generally considered the threshold for initiating treatment in asymptomatic individuals 1
- For TSH between 4.5-10.0 mIU/L, treatment decisions are more controversial and should consider other factors such as symptoms, age, and presence of thyroid antibodies 1, 3
Clinical Approach to Abnormal TSH
- Confirm abnormal value with repeat testing
- Check free T4 levels
- Consider thyroid peroxidase antibody testing
- Consider patient's age, symptoms, and other clinical factors
- Monitor TSH periodically if not treating, as approximately 2-5% of patients with subclinical hypothyroidism progress to overt hypothyroidism annually 1
Remember that TSH values must be interpreted in clinical context, and laboratory reference ranges may vary slightly between institutions.