Normal TSH Reference Range
The normal reference range for serum TSH in adults is 0.4–4.5 mIU/L, based on disease-free populations from the NHANES III study. 1
Standard Adult Reference Range
The American Association of Clinical Endocrinologists recommends a TSH reference range of 0.45–4.5 mIU/L for adults, with this range derived from NHANES III data analyzing disease-free populations. 1
Most laboratories define abnormal TSH using the 95% reference interval for their particular assay, generally about 0.4–4.5 mIU/L, though laboratories use varying types of assays. 2
The geometric mean TSH in disease-free populations is approximately 1.4 mIU/L, serving as a key reference point for evaluating thyroid function. 3
Age-Related Considerations
TSH reference ranges shift upward with advancing age, making standard population reference ranges potentially inappropriate for older adults. 2, 3
Approximately 12% of persons aged 80 years or older with no evidence of thyroid disease have TSH levels greater than 4.5 mIU/L, indicating that age-adjusted reference ranges should be considered in elderly patients. 2, 3
The upper limit of the TSH reference range may reach 7.5 mIU/L in patients over 80 years of age. 3
For elderly patients (>80–85 years) with elevated serum TSH ≤10 mIU/L, a wait-and-see strategy is generally recommended, avoiding hormonal treatment. 4
Pregnancy-Specific Ranges
During pregnancy, particularly the first trimester, TSH levels are physiologically lower than non-pregnant reference ranges. 2
Pregnancy can cause physiologically low TSH with normal free hormone levels, especially during the first trimester. 1
For pregnant women or those planning pregnancy, TSH should be targeted at <2.5 mIU/L in the first trimester to prevent adverse pregnancy outcomes. 3
Important Caveats About TSH Interpretation
TSH secretion is highly variable and sensitive to multiple non-thyroidal factors, making single measurements unreliable for diagnosis. 2
Serum TSH levels can vary by as much as 50% of mean values on a day-to-day basis, with up to 40% variation in serial TSH measurements performed at the same time of day. 2
TSH secretion is frequently suppressed during phases of acute illness, and levels may be affected by medications including iodine, dopamine, glucocorticoids, octreotide, or bexarotene. 2, 1
Adrenal insufficiency, anorexia nervosa, certain autoimmune diseases, and pituitary adenomas can interfere with normal circulating TSH levels. 2
Serial TSH measurements are essential for establishing that a thyroid disorder is real and persistent—never rely on a single TSH value to establish a diagnosis. 2
Laboratory-Specific Variations
Each laboratory should establish its own reference intervals using the specific immunoassay platform employed, as reference intervals vary significantly between different manufacturer assays. 5
The TSH assay used should have a functional sensitivity of at least 0.02 mIU/L, independently established by each laboratory. 1
Laboratories must engage in appropriate quality control procedures to ensure accurate and reproducible results. 1
Clinical Thresholds for Action
Values below 0.1 mIU/L are generally considered low, while values above 6.5 mIU/L are considered elevated in screening contexts. 1
TSH >10 mIU/L with normal free T4 indicates severe subclinical hypothyroidism requiring treatment regardless of symptoms, as this carries approximately 5% annual risk of progression to overt hypothyroidism. 1, 3
TSH 4.5–10 mIU/L with normal free T4 represents mild subclinical hypothyroidism where treatment decisions should be individualized based on symptoms, antibody status, and patient characteristics. 1, 3
TSH <0.45 mIU/L warrants repeat testing with free T4 to distinguish subclinical from overt hyperthyroidism. 1