Difference Between TSH and Highly Sensitive TSH
The main difference between standard TSH and highly sensitive TSH is that highly sensitive TSH assays can detect much lower concentrations of TSH, allowing for better detection of hyperthyroidism and subclinical thyroid disorders.
Technical Differences
Standard TSH tests: Earlier TSH assays had limited sensitivity at the lower end of the measurement range, making it difficult to distinguish between normal and suppressed TSH levels.
Highly sensitive TSH tests: These use advanced "sandwich" immunoassay techniques that provide:
- Lower detection limits (can detect TSH levels <0.1 mIU/L)
- Greater precision at the lower end of the measurement range
- Ability to differentiate between normal and suppressed TSH levels 1
Clinical Applications
Advantages of Highly Sensitive TSH
- Superior detection of hyperthyroidism: Can reliably detect suppressed TSH levels characteristic of hyperthyroidism, with sensitivity of approximately 98% 2
- Better monitoring of thyroid replacement therapy: Can detect over-replacement with levothyroxine by identifying suppressed TSH levels 3
- Front-line assessment of thyroid function: Can serve as an initial screening test for thyroid dysfunction 4
- Prediction of TRH stimulation response: Basal TSH measurements with sensitive assays can predict TSH response to thyrotropin-releasing hormone stimulation at least 95% of the time 1
Performance Criteria for Highly Sensitive TSH Assays
For an assay to be considered truly "sensitive," it should meet these criteria:
- The overlap between the assay detection limit and the lower limit of normal should be less than 1%
- Basal TSH measurements should predict TRH stimulation response at least 95% of the time
- Basal TSH measurements should be 95% sensitive and 95% specific for detecting hyperthyroidism 1
Clinical Limitations and Considerations
Despite their advantages, highly sensitive TSH assays have important limitations:
False positives in hospitalized patients: In hospitalized patients, up to 17.2% may have abnormal TSH results, with only 24% of those with undetectable TSH actually having thyroid disease 5
Non-thyroidal illness interference: Severe non-thyroidal illness can suppress TSH independent of thyroid status 6
Medication effects: Glucocorticoids and other medications can suppress TSH, leading to false positive results 5, 6
Different reference ranges needed: Different reference intervals for TSH may be needed for hospitalized versus non-hospitalized patients 5
Central hypothyroidism: A suppressed TSH may be seen in central hypothyroidism, which could lead to misdiagnosis if only TSH is measured 6
Practical Application
When interpreting TSH results:
- For screening asymptomatic adults, the highly sensitive TSH test is the preferred initial test 2
- In hospitalized patients with non-thyroidal illness, TSH results should be interpreted with caution 5
- When monitoring thyroid hormone replacement therapy, highly sensitive TSH provides similar information to TRH stimulation tests 3
- Multiple TSH measurements over time provide more reliable diagnostic information than a single test 7
Reference Ranges
- General reference range for TSH: approximately 0.4 to 4.5 mIU/L 2
- Values for serum TSH below 0.1 mU/L are considered low 2
- Values above 6.5 mU/L are considered elevated 2
Remember that TSH levels can vary by subpopulations (race/ethnicity, sex, age), with up to 12% of persons aged 80 years or older having TSH levels greater than 4.5 mIU/L without evidence of thyroid disease 2.