Test Overview
Test Methodology

Competitive Immunoassay

Test Usage

Thyroid function test, for the diagnosis of hypothyroidism or hyperthyroidism in conjunction with T4 result.

Reference Range *

22 - 37% Uptake

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

Test Limitations

Alterations in binding capacity or quantity of TBG may alter the free thyroid hormone level in blood. T3 uptake INCREASE: therapy with anticoagulants, salicylates, androgenic hormone, kidney disease, early infancy 1-9 weeks, metastatic malignancy, patients with abnormal serum protein patterns (e.g., nephrosis, hepatitis, cirrhosis), prednisone, butrazolidin, phenytoin, penicillin (large doses), polycythemia vera, supraventricular tachycardia (atrial arrhythmia), severe pulmonary insufficiency with CO2 retention, T3 uptake DECREASE: pregnancy, oral contraceptives, with administration of estrogen, Enovid, Premarin or Norlutin.

Test Details
Days Set Up
Daily, 24 hours
Analytic Time

4 hours

Soft Order Code
T3U
MiChart Code
T3 Uptake and FTI
Synonyms
  • Free Thyroxine Index
  • FTI
  • T7
  • Thyroxine Index, Free
  • Triiodothyronine Uptake
  • T3U
  • FREE THYROXINE INDEX
  • T3 UPTAKE
Laboratory
Chemical Pathology
Section
Automation
Specimen Requirements
Collection Instructions

Collect specimen in a red top or SST tube. Centrifuge, aliquot serum into a plastic vial and refrigerate. Test should be ordered in conjunction with a total T4.

Alternate Specimen
Red Top Tube
Rejection Criteria
Anticoagulants not acceptable.
Contraindications
This test should not be ordered alone, it is only useful in conjunction with a total T4.
Yellow Top Tube
Normal Volume
0.5 mL serum
Minimum Volume
0.25 mL serum
Additional Information

The T7 or estimated free thyroxine index is a mathematical approximation of free thyroxine concentration in blood. It is calculated as follows: T7 equals T3u/100 X T4 mcg/dl. In the presence of thyroid binding globulin abnormalities the free thyroxine index is a valuable laboratory indicator of clinical thyroid status. This is an indirect estimate of thyroid binding globulin capacity and actual measurement of thyroid binding globulin may be desirable to differentiate between quantitative versus qualitative abnormalities in thyroid binding globulin.

Billing
CPT Code
84479
Fee Code
32021
LOINC
3050-2
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