Test Overview
Competitive Immunoassay
Thyroid function test, for the diagnosis of hypothyroidism or hyperthyroidism in conjunction with T4 result.
22 - 37% Uptake
* Reference ranges may change over time. Please refer to the original patient report when evaluating results.
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
4 hours
- Free Thyroxine Index
- Thyroxine Index, Free
- Triiodothyronine Uptake
Specimen Requirements
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.
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.