Normal TSH Levels in Healthy Adults
The normal TSH reference range for healthy adults is 0.4-4.5 mIU/L, based on the most rigorous population screening studies.
Standard Reference Range
The widely accepted TSH reference range is 0.4-4.5 mIU/L, established by the NHANES III study of over 13,000 disease-free individuals, representing the 2.5th to 97.5th percentile in a population rigorously screened to exclude thyroid disease, thyroid antibodies, and medications affecting thyroid function 1, 2.
Most laboratories define normal TSH using the 95% reference interval for their particular assay, generally about 0.4 to 4.5 mIU/L 3.
The geometric mean TSH concentration in healthy populations is approximately 1.4 mIU/L 1, 2.
Important Clinical Context
Population-Specific Variations
TSH reference ranges can vary by population, ethnicity, and age, making population-specific reference intervals important for accurate diagnosis 3.
Studies from different populations show varying upper limits: Brazilian adults showed 0.56-4.45 mIU/L 4, Finnish adults 0.4-3.4 mIU/L with intensive screening 5, and Turkish adults 0.38-4.22 mIU/L 6.
African-Americans with very low incidence of Hashimoto thyroiditis have a mean TSH level of 1.18 mIU/L, suggesting this may represent the true normal mean for populations without thyroid autoimmunity 7.
Age-Related Considerations
TSH secretion varies significantly with age—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 the "standard" population reference interval is probably inappropriate for older adults 3.
TSH levels tend to be lower in subjects older than 50 years compared to younger individuals in some populations 8.
Gender Differences
- TSH levels show minimal clinically significant differences between males and females, though some studies report slightly higher median TSH in females 6.
Critical Caveats for Clinical Use
Measurement Variability
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 3, 9.
A single TSH value should never be used to establish a diagnosis of thyroid dysfunction—serial TSH measurements are essential to confirm that a thyroid disorder is real and persistent 3.
Factors That Interfere with TSH
TSH secretion is highly sensitive to factors other than thyroid disorders: serum TSH is frequently suppressed during acute illness 3.
Medications and substances that affect TSH include iodine, dopamine, glucocorticoids, octreotide, and bexarotene 3.
Adrenal insufficiency, pregnancy (particularly first trimester), anorexia nervosa, certain autoimmune diseases, and pituitary adenomas can interfere with normal circulating TSH levels 3.
Laboratory Assay Differences
- Laboratories use varying types of assays, and the threshold for abnormal TSH is arbitrary—not based on risk for adverse health outcomes but simply on normal population distribution 3.
Clinical Interpretation Algorithm
TSH 0.45-4.5 mIU/L indicates normal thyroid function in most adults 1, 2.
TSH <0.45 mIU/L suggests subclinical or overt hyperthyroidism—measure free T4 and T3 to distinguish between the two 1.
TSH 4.5-10 mIU/L indicates subclinical hypothyroidism—measure free T4 and consider thyroid antibodies 1.
TSH >10 mIU/L generally warrants treatment consideration, particularly if symptomatic 1.
Confirm all abnormal values with repeat testing before making treatment decisions, as TSH secretion is pulsatile and variable 1, 2.