T3 uptake (T3U) is a thyroid function test for the diagnosis of hypothyroidism or hyperthyroidism. It is used with thyroxine (T4) to provide a free T4 index (FTI). T3 uptake is not a measurement of serum T3. Rather, it is an indirect measure of the quantity of available thyroxine binding proteins (thyroxine binding globulin, prealbumin, and albumin) in plasma. T3U is directly proportional to the degree of saturation of the binding proteins by thyroxine.
T3U is not useful alone but should be combined with T4 to calculate the FTI (also called T7). FTI is calculated is calculated by multiplying T4 by T3U and dividing the product by 100.
T7= (T4 x T3-Uptake)/100
T3U is decreased during pregnancy, with supra-normal doses of estrogen, early in acute hepatitis, and with genetic TBG excess. It is increased by anabolic hormones and glucocorticoids, the nephrotic syndrome, and genetic TBG deficiency.
T3U is measured with an electrochemiluminescence immunoassay in which exogenous T4 is added to saturate thyroid binding globulin. Reference range is age dependent. The adult reference range is 24-39%.
Specimen requirement is one red-top gel barrier tube of blood.
Most clinical laboratories no longer recommend ordering T3U. Newer methods for direct measurement of free T4 has eliminated the need to estimate free hormone levels with T3U and FTI. Thyroid stimulating hormone (TSH) and free thyroxine (Free T4) are preferred for thyroid disorder screening.
References
Gruhn JG, Barsano CP, Kumar Y. The development of tests of thyroid function. Arch Pathol Lab Med. 1987 Jan; 111(1):84-100.
Ross DS, et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016;26(10):1343-1421.
Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Golfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
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