Total Triiodothyronine (T3)
Total T3 measures all circulating triiodothyronine, the metabolically active thyroid hormone. The median total T3 for US adults is approximately 117.0 ng/dL, with values declining with age.
Unit: ng/dL · 12 slices · age and sex · 1 source
Filed under panels: Thyroid Panel · topics: Endocrine
Triiodothyronine (T3) is the more potent and metabolically active thyroid hormone. Most circulating T3 is produced by peripheral conversion of T4 to T3 in tissues rather than direct thyroid secretion. Total T3 includes both protein-bound and free fractions. T3 levels decline more steeply with age than T4, reflecting reduced peripheral conversion. These population distributions exclude participants with self-reported thyroid disease or current thyroid medication use.
Population Distribution
Browse by Demographic
| Age (years) | male (ng/dL) | female (ng/dL) |
|---|---|---|
| 20-29 | 92–161.1 (120) | 90.7–170.5 (116.7) |
| 30-39 | 89.6–157.9 (117) | 89.4–154 (115.5) |
| 40-49 | 88.9–150.8 (117) | 81–163.9 (109) |
| 50-59 | 81.3–143.9 (112.6) | 81.5–139 (111) |
| 60-69 | 83.1–145 (110) | 83.2–140 (111) |
| 70+ | 74–135.9 (100) | 77–133.8 (104) |
Frequently Asked Questions
How is T3 related to T4?
The thyroid secretes mostly T4, which is then converted to the more active T3 in peripheral tissues (liver, kidney, muscle). T3 is approximately four times more potent than T4 per unit weight. TSH regulates T4 production; T3 levels reflect both thyroid output and peripheral conversion capacity.
Why does total T3 decline more with age than total T4?
Peripheral T4-to-T3 conversion decreases with age, particularly in chronic illness and nutritional restriction. Even in adults without self-reported thyroid disease, conversion efficiency is reduced in older age groups, contributing to lower T3 despite relatively preserved T4.
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.