From the Guidelines
Thyroid-stimulating hormone (TSH) reference ranges vary by age, with newborns having the widest range and adults having narrower ranges. For newborns, the normal TSH range is approximately 1.0-39.0 mIU/L in the first week, which gradually decreases to 0.4-7.0 mIU/L by one month of age 1. Infants and children typically have TSH ranges of 0.7-6.0 mIU/L. For adults, the standard reference range is generally 0.4-4.5 mIU/L, though some laboratories may use slightly different ranges (0.5-5.0 mIU/L) 2. In older adults over 65, the upper limit may increase slightly to around 5.0-6.0 mIU/L, as 12% of persons aged 80 years or older with no evidence of thyroid disease have been found to have TSH levels greater than 4.5 mIU/L 1. These age-specific ranges are important because thyroid function naturally changes throughout life. TSH levels outside these ranges may indicate thyroid disorders, but interpretation should always consider the patient's age, symptoms, and other thyroid hormone measurements. During pregnancy, TSH ranges are typically lower, with first trimester values of 0.1-2.5 mIU/L, second trimester 0.2-3.0 mIU/L, and third trimester 0.3-3.0 mIU/L, reflecting the body's increased metabolic demands. Some key points to consider when interpreting TSH levels include:
- The reference interval for a particular assay can vary, with a general range of about 0.4 to 4.5 mIU/L 1
- TSH secretion varies among different subpopulations, such as those defined by race/ethnicity, sex, and age 1
- TSH levels can be affected by factors other than thyroid disorders, such as acute illness, certain medications, and adrenal insufficiency 1
- Serial TSH measurements are essential to establish a diagnosis of thyroid dysfunction, as TSH levels can vary by as much as 50% of mean values on a day-to-day basis 1. It is also important to note that the effectiveness of early detection and treatment of thyroid dysfunction is still a topic of debate, with some studies suggesting that treatment of subclinical hypothyroidism may not have a significant impact on morbidity or mortality 3. However, other studies have found that subclinical hypothyroidism is associated with poor obstetric outcomes and poor cognitive development in children 2, 4. Overall, the interpretation of TSH levels should be done with caution, taking into account the patient's age, symptoms, and other thyroid hormone measurements, as well as the potential risks and benefits of treatment.
From the Research
TSH Reference Range by Age
- The reference range for TSH varies by age, with the 2.5th percentile consistent across age groups at approximately 0.5 mU/l 5.
- The 97.5th percentile increases from age 40 upwards, with the reference range upper limit being 3.75 mU/l at age 40 and 5.0 mU/l at age 90 5.
- The use of age-specific upper limits reclassifies only 0.1-1.9% of participants as normal or abnormal compared to a common cut-off of 4.0 mU/l, except in participants aged 85 years or more, where reclassification rates are higher (2.1-4.7%) 5.
Age-Specific TSH Reference Ranges
- Studies have shown that age-specific TSH reference ranges have minimal impact on the diagnosis of thyroid dysfunction, except in the very old 5.
- Other studies have focused on the treatment of hypothyroidism, including the use of levothyroxine monotherapy versus combination therapy, but do not provide specific information on TSH reference ranges by age 6, 7, 8, 9.
Interassay Differences in TSH Measurement
- Different TSH assays may show intermethod differences in performance, particularly at high-normal TSH concentrations, which can affect clinical decision-making 5.