Normal TSH Range in Adults
The normal TSH reference range for adults is 0.45-4.5 mIU/L, representing the 2.5th to 97.5th percentile in disease-free populations. 1
Standard Reference Range
The widely accepted TSH reference range of 0.45-4.5 mIU/L was established by the NHANES III study of over 13,000 disease-free individuals and applies across adult age groups 1
This range was derived from populations screened to exclude thyroid disease, thyroid antibodies, and medications affecting thyroid function 1
The geometric mean TSH concentration in healthy populations is approximately 1.4 mIU/L 1
Clinical Interpretation of TSH Values
TSH 0.45-4.5 mIU/L indicates normal thyroid function 1
TSH <0.45 mIU/L suggests subclinical or overt hyperthyroidism and requires measurement of free T4 and T3 to distinguish between these conditions 1
TSH 4.5-10 mIU/L indicates subclinical hypothyroidism and warrants measurement of free T4 and consideration of thyroid antibodies 1
TSH >10 mIU/L generally warrants treatment consideration, particularly if the patient is symptomatic 1
Controversy Regarding Upper Limit
While some experts have argued for a narrower optimal range of 0.4-2.5 mIU/L based on data suggesting that most individuals without thyroid disease have TSH below 2.5 mIU/L 2, 3, the current clinical consensus maintains the 0.45-4.5 mIU/L range for practical purposes 4. Classifying patients with TSH between 2-4 mIU/L as abnormal and treating them with levothyroxine is likely doing more harm than good 4.
Age-Related Considerations
The TSH reference range shifts upward with advancing age, with the upper limit reaching approximately 7.5 mIU/L in patients over 80 years 5
Older adults (≥85 years) are twice as likely to be started on levothyroxine compared to those aged 65-69 years 1
Critical Caveats Before Diagnosis
A single abnormal TSH should never trigger immediate diagnosis or treatment due to high TSH variability and frequent spontaneous reversion to normal 1
Confirm abnormal values with repeat testing after 3-6 weeks before making treatment decisions, as 30-60% of mildly abnormal TSH levels normalize spontaneously 5, 1
Exclude non-thyroidal causes of abnormal TSH including acute illness, medications, recent iodine exposure, and recovery from thyroid treatment before diagnosing thyroid dysfunction 1
TSH secretion is pulsatile and variable, making single measurements potentially misleading 1
TSH Sensitivity and Specificity
- TSH has a sensitivity above 98% and specificity greater than 92% for detecting thyroid dysfunction, making it the most reliable screening test for thyroid disease 5