Test Overview

Test Methodology

Chemiluminescent Immunoassay

Test Usage

Thyroid function test, useful in the diagnosis of hypothyroidism or hyperthyroidism.

Reference Range*

4.9 - 12.0 mcg/dL

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Limitations

Specimen may be stored at room temperature up to 8 hours. [12/05]

Alterations in binding capacity or quantity of TBG (thyroxine binding globulin) may increase or decrease total thyroxine without causing symptoms. T4 is INCREASED with pregnancy, estrogens, oral contraceptives, infectious hepatitis, newborns, hyperproteinemia, porphyria, D-thyroxine therapy. Most common cause of elevated T4 in nonthyroidal disease is said to be liver disease. T4 is DECREASED by therapy with anticoagulants, salicylates, glucocorticoids, hypoproteinemia, phenytoin, triiodothyronine therapy, etc. Therefore T3 uptake should be ordered also. If nonthyroidal factors may be present. Normal (euthyroid) range will be higher in patients taking thyroxine as medication. Less sensitive than TSH in diagnosis of hypothyroidism.

Test Details

Days Test Performed

Daily, 24 hours

Analytic Time

4 hours

Soft Order Code


MiChart Code





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Chemical Pathology

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Specimen Requirements

Offsite Collection Instructions

Collect specimen in a red top or SST tube. Centrifuge, aliquot serum into a plastic vial and refrigerate up to 48 hours or freeze for longer storage.

Rejection Criteria

Anticoagulants not acceptable.


Yellow Top Tube
Red Top Tube

Normal Volume

0.5 mL serum

Minimum Volume

0.25 mL serum

Additional Information

The combination of the serum T4 and TBG index allows one to determine whether an abnormal T4 result is due to alterations in serum thyroxine-binding globulin concentrations or thyroid hormone production rate. Deviation from normal in the same direction by each test usually indicates that the abnormal T4 is due to increased thyroid hormone production. Deviation from normal in opposite directions by each test generally means that the abnormal T4 is due to an alteration in TBG concentration. Cause of increased TBG (which may show increased T4 and decreased TBG index) are: pregnancy, estrogens, infectious hepatitis, acute intermittent porphyria and hereditary TBG deficiency. Salicylates and diphenylhydantoin which displace T4 from TBG may lower T4 by 20-30%, and TBG index may be normal or slightly increased.

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