Free Thyroxine (T4)
Free T4 measures the biologically active, unbound fraction of thyroxine. Across US males aged 30-39, the median free T4 in NHANES 2009-2012 is 0.8 ng/dL, with a narrower distribution than total T4.
Unit: ng/dL · 12 groups · age and sex · 1 source
Filed under panels: Thyroid Panel · topics: Endocrine
Free Thyroxine (T4) by age and sex
| Age (years) | Males (ng/dL) | Females (ng/dL) |
|---|---|---|
| 20-29 | median 0.8 middle 90% 0.7 to 1.1 | median 0.8 middle 90% 0.7 to 1 |
| 30-39 | median 0.8 middle 90% 0.6 to 1 | median 0.8 middle 90% 0.6 to 1 |
| 40-49 | median 0.8 middle 90% 0.6 to 1 | median 0.8 middle 90% 0.6 to 1 |
| 50-59 | median 0.8 middle 90% 0.6 to 1 | median 0.8 middle 90% 0.6 to 1.1 |
| 60-69 | median 0.8 middle 90% 0.6 to 1.1 | median 0.8 middle 90% 0.6 to 1 |
| 70+ | median 0.8 middle 90% 0.6 to 1.1 | median 0.8 middle 90% 0.6 to 1.1 |
Download Free Thyroxine (T4) percentiles (CSV) Values at full stored precision; tables on this site may round for display. Licensed under CC BY 4.0.
Population Distribution
Free thyroxine (free T4) is the small fraction of T4 not bound to carrier proteins and available to enter cells and exert biological effects. It is a more direct measure of thyroid hormone activity than total T4 because it is not affected by changes in binding protein levels. Free T4 is typically measured alongside TSH for thyroid function assessment. These population distributions exclude participants with self-reported thyroid disease or current thyroid medication use.
Frequently Asked Questions
Why is free T4 preferred over total T4 in many clinical contexts?
Free T4 is not affected by binding protein levels, which can change with pregnancy, oral contraceptive use, and liver disease. This makes free T4 a more reliable indicator of actual thyroid hormone activity in those situations.
Why is the free T4 distribution narrower than total T4?
The free fraction is tightly regulated by the pituitary-thyroid axis. Small deviations from the set point trigger TSH responses that restore free T4 toward the individual's baseline. Total T4 has more variability because it is also influenced by binding protein fluctuations that the feedback loop does not correct.
Why are males and females shown separately for free T4?
Sex differences in free T4 are smaller than for TSH, but females have higher rates of subclinical thyroid autoimmunity, which can subtly shift the distribution. Showing separate percentiles avoids combining groups with different underlying distributions.
Data Sources
Cite this data
LabNorms. Free Thyroxine (T4) population percentiles by age and sex. Data: NHANES 2009-2012 Thyroid Profile (Subsample A). https://labnorms.com/analytes/free-t4/. Licensed under CC BY 4.0. Accessed [date].
Download Free Thyroxine (T4) percentiles (CSV) Values at full stored precision; tables on this site may round for display. Licensed under CC BY 4.0.