Mobile Site ›
Normal View

Test ID: TUP
T3 (Triiodothyronine), Uptake, Serum

Secondary ID A test code used for billing and in test definitions created prior to November 2011


NY State Approved Indicates the status of NY State approval and if the test is orderable for NY State clients.


Useful For Suggests clinical disorders or settings where the test may be helpful

Estimating the amount of circulating free thyroxine, when in conjunction with total thyroxine results to calculate the free thyroxine index

Method Name A short description of the method used to perform the test

Chemiluminometric Immunoassay

Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name

T3 (Triiodothyronine), Uptake, S

Aliases Lists additional common names for a test, as an aid in searching

T3U, Serum
Triiodothyronine Uptake (T3U), Serum
T3 Uptake, S

Specimen Type Describes the specimen type needed for testing


Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.


Preferred: Serum gel

Acceptable: Red top

Specimen Volume: 0.5 mL

Collection Instructions:

1. Serum gel tubes should be centrifuged within 2 hours of collection.

2. Red-top tubes should be centrifuged and aliquoted within 2 hours of collection.

Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.

0.3 mL

Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected


Mild OK; Gross reject








Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.

Specimen TypeTemperatureTime
SerumRefrigerated (preferred)7 days
 Frozen 30 days
 Ambient 8 hours

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.

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.

Males: 27-37%

Females: 20-37%

Interpretation Provides information to assist in interpretation of the test results

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

-Acute hepatitis

-Drugs that prevent ovulation


Conditions that elevate T-uptake values:

-Protein malnutrition

-Chronic liver disease

-Nephrotic syndrome

-Phenytoin therapy

-Heparin therapy


-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.

Clinical Reference 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

Method Description Describes how the test is performed and provides a method-specific reference

This is a chemiluminometric immunoassay. Triiodothyronine (T3)-uptake is measured using an automated sandwich 2-site chemiluminometric immunoassay on the Advia Centaur instrument. The specimen is incubated with acridinium ester (AE)-labeled T3-BGG (bovine gamma globulin) and unlabeled T3. The unlabeled T3 fills available thyroid-binding sites in the specimen; and the AE-labeled T3-BGG does not bind to the binding proteins in the specimen. The AE-labeled T3-BGG and the remaining unlabeled T3 then compete for binding sites on a monoclonal mouse anti-T3 antibody in the solid phase. A greater amount of unlabeled T3 bound to the binding proteins in the specimen results in more T3-BGG-AE binding to the monoclonal antibody, an indication of a greater amount of unsaturated binding proteins. A direct relationship exists between the concentration of unsaturated binding proteins in a specimen and the relative light units (RLUs) detected by the system.(Package insert: Bayer Advia: Centaur TUP, 2004-03. Bayer Diagnostics, East Walpole, MA 02032)

PDF Report Indicates whether the report includes an additional document with charts, images or other enriched information


Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.

Monday through Friday, Sunday; 6 a.m.-3 p.m.

Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.

Same day/1 day

Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result

2 days

Specimen Retention Time Outlines the length of time after testing that a specimen is kept in the laboratory before it is discarded

1 month

Performing Laboratory Location The location of the laboratory that performs the test


Test Classification Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.

This test has been cleared or approved by the U.S. Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.


LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.

Result IDReporting NameLOINC Code
TUPT3 (Triiodothyronine), Uptake, S3050-2