Normal TSH Levels
The normal TSH reference range is 0.45-4.5 mIU/L for adults across all age groups, based on the NHANES III study of over 13,000 disease-free individuals. 1, 2
Standard Reference Range
TSH values between 0.45-4.5 mIU/L indicate normal thyroid function and represent the 2.5th to 97.5th percentile in 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, which represents the true center of normal thyroid function. 1
This reference range is endorsed by the American Association of Clinical Endocrinologists and applies consistently across adult age groups. 2
Clinical Interpretation Framework
When evaluating TSH results, use the following algorithmic approach:
TSH 0.45-4.5 mIU/L: Normal thyroid function; no further testing needed unless clinically indicated. 1, 2
TSH <0.45 mIU/L: Suggests subclinical or overt hyperthyroidism; measure free T4 and T3 to distinguish between these conditions. 1, 2
TSH 4.5-10 mIU/L: Indicates subclinical hypothyroidism; measure free T4 and consider checking thyroid antibodies to assess for autoimmune thyroiditis. 1
TSH >10 mIU/L: Generally warrants treatment consideration, particularly if the patient is symptomatic or has other risk factors. 1
Critical Caveats and Pitfalls
A single abnormal TSH value should never trigger immediate diagnosis or treatment due to high TSH variability and frequent spontaneous reversion to normal without intervention. 1
Always confirm abnormal values with repeat testing before making treatment decisions, as TSH secretion is pulsatile and exhibits significant intra-individual variability. 1
Multiple tests should be performed over a 3- to 6-month interval to confirm or rule out abnormal findings before initiating therapy. 3
Exclude non-thyroidal causes before diagnosing thyroid dysfunction:
Acute illness, medications (dopamine, glucocorticoids, dobutamine, amiodarone), and recovery from thyroid treatment can all cause abnormal TSH values without true thyroid disease. 1, 2
Non-thyroidal illness can cause low TSH, low T3, and normal/low T4 without actual thyroid dysfunction. 2
Pregnancy can cause physiologically low TSH with normal free hormone levels. 2
Heterophilic antibodies in the TSH assay may produce spuriously elevated TSH values. 4
Age-Related Considerations
In older adults (age 70-79 years), the reference range becomes slightly broader (0.4-5.9 mIU/L), though the standard 0.45-4.5 mIU/L range remains widely applicable. 4
Patients ≥85 years are twice as likely to start levothyroxine compared to those aged 65-69 years, suggesting potential overtreatment in the elderly population. 1
Controversy Regarding Upper Limit
While some research has suggested a narrower optimal TSH range of 0.4-2.5 mIU/L 5, 6, there is insufficient justification to lower the upper normal limit from 4.5 mIU/L in clinical practice. 7