T3 Uptake

(T3) Uptake

CPT Test code: 84479

Related Information:
Specimen: Serum
Volume: 1 mL (adult), 0.8 mL (pediatric)
Minimum Volume: 0.5 mL (adult), 0.3 mL (pediatric) (Note: This volume does not allow for repeat testing.)
Container: Red-top tube or gel-barrier tube
Special Instructions: This test reflects assessment of thyroxine-binding globulin (TBG) and should not be ordered alone.
Collection: If a red-top tube is used, transfer separated serum to a plastic transport tube.
Storage Instructions: Refrigerate
Reference Interval: See table.1

Age Male

(%)

Female

(%)

0-11 mo 23-34 23-36
1-3 y 24-35 24-36
4-6 y 24-34 24-35
7-12 y 24-33 22-35
11-15 y 25-37 23-37
16-18 y 24-38 23-35
>18 y 24-39 24-39
Use: Thyroid function test for the diagnosis of hypothyroidism or hyperthyroidism, used with thyroxine (T4) or equivalent to provide free T4 index, FTI. An indirect measure of binding protein, the T3 uptake reflects available binding sites (ie, reflects TBG). T3 uptake is not a measurement of serum T3. It should never be used alone; rather, its usual application is use with thyroxine (T4).
Limitations: Alterations in binding capacity of TBG are described with major illness and with high doses of salicylates and corticosteroids; with use of heroin, methadone, phenytoin, and perphenazine. Alterations occur with malnutrition, such as in metastatic malignancy, and are found in patients with abnormal serum protein patterns (eg, nephrotic syndromes, cirrhosis). Other states in which changes in TBG occur include infancy, acromegaly, molar and ordinary pregnancy, oral contraceptives, and with exogenous hormones including androgens, anabolic steroids, and estrogens. Hereditary increase and decrease of TBG occurs.
Footnotes: 1. Soldin SJ, Cook J, Beatey J, et al, “Pediatric Reference Ranges for Thyroxine and Tri-iodothyronine Uptake,” Clin Chem, 1992, 38:960.
References: Bakerman S, A, B, C’s of Interpretive Laboratory Data, Greenville, NC: Interpretive Laboratory Data Inc, 1984.

Burke MD, “Thyroid Function Studies. Test Strategies and Interpretation of Results,” Postgrad Med, 1980, 68(6):169-77. PubMed 6776511

Gruhn JG, Barsano CP, Kumar Y, “The Development of Tests of Thyroid Function,” Arch Pathol Lab Med, 1987, 111(1):84-100.PubMed 3541847