The triiodothyronine uptake assay (T3U) is used as a complementary thyroid function test and assesses the number of free thyroid hormone binding sites on their transport proteins.
Diagnostiki Athinon no longer performs the triiodothyronine uptake assay (T3U). Please see the other thyroid function tests.
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Triiodothyronine (T3) and thyroxine (T4) hormones bind to plasma proteins, mainly to thyroxine-binding or thyroid-binding globulin (TBG) and, to a lesser extent, 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 determined after all available free binding sites with TBG have been previously blocked. This rate is inversely proportional to the saturation rate of TBG. A higher intake of triiodothyronine means less TBG available, possibly due to hyperthyroidism.
Determination of triiodothyronine uptake is not used as a single test; it is used along with T4 measurement to determine the 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, and warfarin.
- Decrease: Acute hepatitis, congenital TBG excess, estrogen-secreting tumors, hypothyroidism, pregnancy. Medications: Antithyroid drugs, clofibrate, estrogen, oral contraceptives, thiazide diuretics.
