Sorted By Test Name - Mayo Medical Laboratories

TUP
81792
Interpretation: Triiodothyronine (T3) values >180 ng/dL in adults or >200 ng/dL in children are
consistent with hyperthyroidism or increased thyroid hormone-binding proteins. Abnormal levels (high or
low) of thyroid hormone-binding proteins (primarily albumin and thyroid-binding globulin) may cause
abnormal T3 concentrations in euthyroid patients.
Reference Values:
> or =1 year: 80-190 ng/dL
Clinical References: 1. Hay ID, Klee GG: Linking medical needs and performance goals: clinical
and laboratory perspectives on thyroid disease. Clin Chem 1993;39:1519-1524 2. Klee GG: Clinical usage
recommendations and analytic performance goals for total and free triiodothyronine measurements. Clin
Chem 1996;42:155-159
T3 (Triiodothyronine), Uptake, Serum
Clinical Information: 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 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 triiodothyronine (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.
Useful For: As an estimate the amount of circulating free thyroxine, when in conjunction with total
thyroxine results to calculate the free thyroxine index
Interpretation: 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 (TBG) levels may have similar effects (see
Cautions).
Reference Values:
Males: 27-37%
Females: 20-37%
Clinical References: 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
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