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Unit Code 800093:
T3 (Triiodothyronine), Total, Serum

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Useful For

Second-order testing for hyperthyroidism in patients with low thyroid

stimulating hormone (TSH) values and normal T4 levels

 

Diagnosis of T3 toxicosis

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 (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

(e.g., 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 Algorithm" in Special Instructions.

Reference Values

> or =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.

 

Abnormal levels (high or low) of thyroid hormone-binding proteins

(primarily albumin and TBG) may cause abnormal T3

concentrations in euthyroid patients.

Cautions

T3 is not a reliable marker for hypothyroidism.

 

T3 is not useful for general screening of the population without a

clinical suspicion of hyperthyroidism.

 

Some patients who have been exposed to animal antigens, either

in the environment or as part of treatment or imaging procedures,

may have circulating antianimal antibodies present. These anti-

bodies may interfere with the assay reagents to produce unreliable

results.

Special Instructions and Forms

• Thyroid Function Algorithm 

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

Key