Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

T3
8613
TUP
81792
Clinical References: Moore WT, Eastman RC: Diagnostic Endocrinology. St, Louis, Mosby, 1990,
pp.182-183
T3 (Triiodothyronine), Total, Serum
Clinical Information: Thyroid hormones regulate a number of developmental, metabolic, and neural
activities throughout the body. The 2 main hormones secreted by the thyroid gland are thyroxine, which
contains 4 atoms of iodine (Thyroxine [T4]), and triiodothyronine (T3). T3 is also produced by conversion
(deiodination) of T4 in peripheral tissues. Circulating levels of T4 are much greater than T3 levels, but T3
is more metabolically active than T4, although its effect is briefer. Thyroid hormones circulate primarily
bound to carrier proteins (eg, thyroid-binding globulin [TBG], albumin); a small fraction circulates
unbound (free). Only the free forms are metabolically active. While both T3 and T4 are bound to TBG, T3
is bound less firmly than T4. Total T3 consists of both the bound and unbound fractions. In
hyperthyroidism both T4 and T3 levels are usually elevated, but in a small subset of hyperthyroid patients
only T3 is elevated (T3 toxicosis). In hypothyroidism T4 and T3 levels are decreased. T3 levels are
frequently low in sick or hospitalized euthyroid patients. See Thyroid Function Ordering Algorithm in
Special Instructions.
Useful For: Second-order testing for hyperthyroidism in patients with low thyroid stimulating hormone
values and normal T4 levels Diagnosis of T3 toxicosis
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.
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