Sorted By Test Name - Mayo Medical Laboratories

FRT3P
600915
FRT3
9404
fluctuations and diurnal cycle are important. J AIDS 1990;3:144-151 14. Paglieroni TG, Holland PV:
Circannual variation in lymphocyte subsets, revisited. Transfusion 1994;34:512-516
T3 (Triiodothyronine), Free and 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, under the influence of
thyroid stimulating hormone, are thyroxine (T4), which contains 4 atoms of iodine, and triiodothyronine
(T3). T3 is also produced by conversion (deiodination) of T4 in peripheral tissues; 20% of T3 occurs from
direct synthesis and 80% occurs from conversion of T4 to T3. Circulating levels of T3 are much lower
than T4 levels, but T3 is more metabolically active than T4, although its effect is briefer. In circulation,
99.7% of T3 is bound to proteins, primarily thyroxine-binding globulin. Free T3 is the metabolically
active form of T3 and accounts for 0.3% of the total T3. In hyperthyroidism, both T4 and T3 levels are
usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated. In
hypothyroidism, T4 and T3 levels are decreased. T3 levels are also often low in sick or hospitalized
euthyroid patients.
Useful For: Triiodothyronine (T3) is a second-order test in follow-up to low thyroid stimulating
hormone values in the evaluation of patients suspected of having hyperthyroidism caused by excess T3
(T3 toxicosis). Free T3 (FT3) levels can be helpful in evaluating patients with altered levels of binding
proteins, such as pregnant patients, patients receiving estrogens and anabolic steroids, and patients with
dysalbuminemia. Some investigators recommend FT3 for monitoring thyroid replacement therapy,
although its clinical role is not precisely defined. FT3 also provides a further confirmatory test for
hyperthyroidism to supplement the thyroxine (T4), sensitive thyrotropin, and total T3 assays.
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. T3 often is low in sick or
hospitalized patients, so it is not a good indicator of hypothyroidism. Abnormal levels of binding proteins
to T3 (primarily albumin and thyroxine-binding globulin) may cause either high or low T3 concentrations
in euthyroid patients. Elevations in free T3 are associated with thyrotoxicosis or excess thyroid
replacement therapy.
Reference Values:
T3, FREE
> or =1 year: 2.0-3.5 pg/mL
T3, TOTAL
> 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), Free, Serum
Clinical Information: Normally triiodothyronine (T3) circulates tightly bound to thyroxine-binding
globulin and albumin. Only 0.3% of the total T3 is unbound (free); but the free fraction is the active form.
In hyperthyroidism, both thyroxine (tetraiodothyronine; thyroxine [T4]) and T3 levels (total and free) are
usually elevated, but in a small subset of hyperthyroid patients (T3 toxicosis) only T3 is elevated.
Generally, free T3 (FT3) measurement is not necessary since total T3 will suffice. However, free T3
levels may be required to evaluate clinically euthyroid patients who have an altered distribution of binding
proteins (eg, pregnancy, dysalbuminemia). Some investigators recommend the free T3 assay for
monitoring thyroid replacement therapy, although its clinical role is not precisely defined.
Useful For: Free triiodothyronine (T3) is a second- or third-level test of thyroid function. It provides
further confirmation of hyperthyroidism, supplementing the tetraiodothyronine (T4), sensitive thyrotropin
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