T3 Resin Uptake (T3U / THBR)
Thyroid marker
T3 Uptake
T3 Resin Uptake (T3U / THBR)
An indirect, legacy measure of how saturated the thyroid-binding proteins are with hormone, also reported as the Thyroid Hormone Binding Ratio (THBR). It does NOT measure T3 levels directly; it moves inversely to the number of unoccupied binding sites on thyroxine-binding globulin. It is used together with Total T4 to calculate the Free T4 Index and correct for binding-protein variation.
PED Notes
T3 Uptake is a binding-protein test, not a thyroid-activity test. When TBG rises (oral oestrogen, pregnancy, some AAS-related hepatic effects) there are more free binding sites, so T3 Uptake falls; when TBG falls (high-dose androgens, nephrotic states) T3 Uptake rises. Read alongside Total T4: a high Total T4 with a low T3 Uptake suggests raised binding proteins rather than hyperthyroidism, whereas both moving in the same direction suggests a true thyroid change. Most modern panels replace this with a direct Free T4, so treat it mainly as an input to the Free T4 Index.
Interpreting T3 Uptake:
- T3 Uptake is inversely related to available binding sites: low uptake usually means high binding proteins, high uptake usually means low binding proteins.
- It has no standalone treatment. Its only practical role is correcting Total T4 for binding changes via the Free T4 Index. Where available, a direct Free T4 (already in this knowledge base) is more reliable and should be preferred.
- If a value looks abnormal, confirm thyroid status with TSH and a direct Free T4 rather than acting on T3 Uptake alone.
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range