Thyroid Biomarkers: T3 Uptake

Optimal Takeaways

The T3 uptake test does not measure T3 but instead reflects the ability of binding proteins to carry T3. It must be assessed alongside T4 levels and is not a standalone test but instead is evaluated with serum T4 levels to determine the free T4 index (FTI). A low T3 uptake with a low T4 is associated with hypothyroidism where thyroid hormone levels are low in general. Low T3 uptake with high T4 is associated with high thyroid binding globulin; high T3 uptake with a high T4 is associated with hyperthyroidism; and a high T3 uptake with low T3 is associated with low thyroid binding globulin. 

Conventional Lab Range: 22.00 – 35.00%         

Optimal Dx’s Optimal Range: 27.00 – 35.00%

Low T3 uptake is associated with hypothyroidism with a low T4; elevated TBG with a high T4; and pregnancy (Dunlap 1990).

High T3 uptake is associated with hyperthyroidism with a high T4, and low TBG with a low T4 (Dunlap 1990).

Overview

T3 uptake, also called T3 resin uptake, is not a measurement of T3 despite its name. Instead, it reflects the binding capacity of circulating TBG, the binding protein that carries thyroid hormones. An increase or decrease in thyroid hormone level may be caused by changes in TBG versus dysfunction of the thyroid itself. Measuring the T3 uptake can help distinguish between the two, though it must be measured in conjunction with total T4 in order to calculate the free thyroxine index (FTI), a corrected value for T4. The T3 uptake does not evaluate thyroid function and should not be used as a standalone test (Dunlap 1990). 

The T3 uptake test may also be referred to as a thyroid hormone binding ratio (THBR) test (Spencer 2017). However, since the availability of free T4 assays, T3 uptake has become somewhat obsolete.

References

Dunlap, Dickson B. “Thyroid Function Tests.” Clinical Methods: The History, Physical, and Laboratory Examinations, edited by H Kenneth Walker et. al., 3rd ed., Butterworths, 1990.

Spencer, Carole A. “Assay of Thyroid Hormones and Related Substances.” Endotext, edited by Kenneth R Feingold et. al., MDText.com, Inc., 20 February 2017.