Total Thyroxine (T4)
Total T4 measures all circulating thyroxine, both protein-bound and free. The median total T4 for US adults is approximately 7.5 µg/dL, with values declining modestly with age in males and remaining relatively stable in females.
Unit: µg/dL · 12 slices · age and sex · 1 source
Filed under panels: Thyroid Panel · topics: Endocrine
Total thyroxine (T4) is the primary hormone secreted by the thyroid gland. More than 99% circulates bound to carrier proteins (mainly thyroxine-binding globulin, TBG). Total T4 reflects both the bound and free fractions. Binding protein levels affect total T4 independently of thyroid function, which is why free T4 is often preferred for assessing thyroid status. These population distributions exclude participants with self-reported thyroid disease or current thyroid medication use.
Population Distribution
Browse by Demographic
| Age (years) | male (µg/dL) | female (µg/dL) |
|---|---|---|
| 20-29 | 5.7–10.3 (7.6) | 6.2–11.6 (7.9) |
| 30-39 | 5.5–9.8 (7.5) | 5.8–11.3 (7.9) |
| 40-49 | 5.7–10.1 (7.4) | 5.7–10.9 (7.7) |
| 50-59 | 5.5–10 (7.3) | 5.9–11.1 (7.8) |
| 60-69 | 5.8–10.9 (7.7) | 5.9–10.7 (8) |
| 70+ | 5.6–10.9 (7.7) | 6.2–10.8 (8.1) |
Frequently Asked Questions
How does total T4 differ from free T4?
Total T4 includes both protein-bound and free hormone. Only free T4 is biologically active. Conditions that alter binding protein levels (pregnancy, oral contraceptives, liver disease) change total T4 without changing actual thyroid activity. Free T4 is a more direct measure of thyroid output.
Why are females shown with higher total T4 than males?
Females generally have higher thyroxine-binding globulin (TBG) levels than males. Since TBG carries most circulating T4, higher TBG directly raises total T4 without necessarily reflecting greater thyroid hormone production or activity.
Why are these population percentiles, not clinical reference ranges?
Clinical reference ranges are derived from a selected population of presumed-healthy individuals and are designed to flag potential pathology. Population percentiles from NHANES describe the distribution in the general US population among adults without self-reported thyroid disease. Both are useful and answer different questions.