The triiodothyronine uptake assay (T3U) is used as a complementary test in the control of thyroid function and assesses the number of free thyroid hormone binding sites on their transport proteins.
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Triiodothyronine (T3) and thyroxine (T4) hormones bind to plasma proteins and mainly to thyroxine-binding or thyroid-binding globulin (TBG) and to a lesser extent to thyroid-binding albumin (TB). Determination of triiodothyronine uptake (T3 Uptake, T3U) measures the number of unoccupied binding sites in these proteins and is an indirect indicator of thyroid status.
During this test, a known amount of labeled triiodothyronine and a resin are added to the patient's blood sample. The tagged T3 will be bound to all available TBG sites. The percentage of labeled T3 remaining unbound and bound to the resin is then determined after all available free binding sites with TBG have been previously blocked. This rate is inversely proportional to the saturation rate of TBG. Higher intake of triiodothyronine means less TBG available, possibly due to hyperthyroidism.
Determination of triiodothyronine uptake is not used as a single test, but along with T4 measurement, it is used to determine free thyroxine index (FTI).
Possible Interpretations of Pathological Values
- Increase: Congenital TBG deficiency, hyperthyroidism, hypoproteinemia, malnutrition, nephrosis, nephrotic syndrome, renal failure. Medications: Anabolic steroids, barbiturates, corticosteroids, furosemide, heparin, phenylbutazone, phenytoin, salicylates, thyroxine, warfarin.
- Decrease: Acute hepatitis, congenital TBG excess, estrogen-secreting tumors, hypothyroidism, pregnancy. Medications: Antithyroid, clofibrate, estrogen, oral contraceptives, thiazide diuretics.
Important Note
Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. Correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".
Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.
At Diagnostiki Athinon we answer any questions you may have about the test you perform in our laboratory and we contact your doctor to get the best possible medical care.
