T₃ Uptake

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).

This test reflects assessment of thyroxine-binding globulin (TBG) and should not be ordered alone.

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

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.

Electrochemiluminescence Immunoassay (ECLIA)