T3 (Triiodothyronine), Uptake, Serum
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Thyroxine (T4) is the main thyroid hormone; it circulates in 2 forms, protein-bound (99.05%) and free (0.05%). Free thyroxine (FT4) is the biologically active form. Both bound and free forms are measured by total T4 (TT4) assays. While TT4 is a relatively reliable indicator of T4 levels in the presence of normal binding proteins, it is not a reliable indicator when binding proteins are abnormal. For example, increases in thyroxine-binding proteins may cause increased TT4 levels despite normal FT4 levels and normal thyroid function. Hence, laboratory tests have been developed to compensate for the presence of abnormal types or quantities of thyroxine-binding proteins. These include the triiodothyronine (T3)-uptake test (also called T uptake), the free thyroxine index (FTI), and FT4 assays.
This test, T3-uptake, reflects the level of thyroid-binding globulin (TBG) that is bound by T4. For example, when TBG concentration is decreased, less TBG is available to bind labeled T3, and more labeled T3 reagent binds to the solid-phase material (increased T3 uptake). This is also the case in hyperthyroidism, where higher levels of T4 are present and bind with the TBG, effectively reducing the TBG available to bind with labeled T3. T3-Uptake and TT4 results are used to calculate the FTI, as an estimate of biologically-active thyroxine (FT4) status.
Factors affecting the accuracy of T3-uptake and FTI include:
-FTI is inaccurate when TBG concentration is very abnormal: underestimates FT4 when binding protein concentrations are low, overestimates when binding protein concentrations are high.
-Abnormal types of binding proteins may cause abnormal results.
-Results are changed by drugs or physical conditions that alter the patient's TBG levels, or drugs that compete with endogenous T4 and T3 for protein-binding sites.
Because of its increased accuracy, the FT4 assay (FRT4 / T4 [Thyroxine], Free, Serum by immunoassay) is the preferred routine test.
Estimating the amount of circulating free thyroxine, when in conjunction with total thyroxine results to calculate the free thyroxine index
Values from this test (triiodothyronine: T3-uptake) are used in conjunction with total thyroxine (T4) measurements to calculate the free thyroxine index (FTI):
-FTI=(T4 concentration) x (% T3-uptake)/100
The FTI is a normalized determination that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins.
Hyperthyroidism causes increased FTI and hypothyroidism causes decreased values.
Many drugs, by competing with endogenous T4 and T3 for protein-binding sites, may cause abnormal T3-uptake and FTI even when no thyroid malfunction is present. Physical conditions that alter thyroid-binding globulin levels may have similar effects (see Cautions).
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Triiodothyronine (T3)-uptake values are a function of thyroid hormone binding capacity of serum.
Conditions that decrease T-uptake values:
-Pregnancy (especially in the last trimester)
-Elevated estrogen levels
-Drugs that prevent ovulation
Conditions that elevate T-uptake values:
-Chronic liver disease
-Large doses of salicylates
Thyroid preparations that maintain normal T3 and thyroxine concentrations can elevate T-uptake values when the dose hormone is excessive and decrease T-uptake values when the dose is inadequate.
Replacement therapy with T3 results in decreased T-uptake values.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Whitley RJ, Meikle AW, Watts NB: Thyroid function. In Tietz Fundamentals of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 1996, pp 645-646
2. Klee GG, Hinz VS: The Ciba Corning ACS:180 Plus. In Immunoassay Automation: An Updated Guide to Systems. Edited by DW Chan, Associated Press, New York, 1996, pp 63-102