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Unit Code 600915:
T3 (Triiodothyronine), Free and Total, Serum

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Useful For

T3 is a second-order test in follow-up to low TSH values in the

evaluation of patients suspected of having hyperthyroidism caused

by excess T3 (T3 toxicosis).

 

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 T4, sensitive thyrotropin, and total T3 assays.

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 (TSH), are thyroxine, which contains 4 atoms of iodine (T4),

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 (TBG). Free T3 (FT3) 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.

Reference Values

Free T3:   >=1 year: 2.0-3.5 pg/mL

Total T3:  >=1 year: 80-190 ng/dL

Interpretation

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

TBG) may cause either high or low T3 concentrations in euthyroid

patients.

 

Elevations in free T3 are associated with thyrotoxicosis or excess

thyroid replacement therapy.

Cautions

T3 is not a reliable marker for hypothyroidism and is not useful for

general screening of the population without a clinical suspicion of

hyperthyroidism.

Clinical Reference

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

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