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Introduction ⬇

Triiodothyronine Uptake (T3 U)

This test is an indirect measurement of unsaturated TBG in blood. This determination, expressed in arbitrary terms, is inversely proportional to the TBG. For this reason, low T3 U levels are indicative of situations that result in elevated levels of TBG uptake. For example, in hypothyroidism, when insufficient T4 is available to produce saturation of TBG, unbound TBG (UTBG) is elevated, and T3 U values are low. Similarly, in pregnant patients or those receiving estrogen, TBG levels are increased proportionately more than are T4 levels, resulting in high levels of UTBG, which are reflected in low T3 U results.

This test should not be ordered alone—it is useful only when T4 is done. It is also used to calculate the T7 or FTI. With the improvements in other thyroid function assays, this test does not provide much utility.

Normal Findings ⬆ ⬇

0.9–1.10 (ratio between patient specimen and the standard control)

25%–35% or 25–37 AU (arbitrary units)

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample. Serum is needed.

  2. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

Clinical Implications ⬆ ⬇

See Table 6.16 for implications of clinical conditions for test results.

Interventions ⬆ ⬇

Pretest Patient Care

  1. See Patient Care for Thyroid Testing.

  2. Explain test purpose and procedure.

  3. Have patient avoid strenuous exercise.

  4. Do not administer radiopaque contrast for 1 week before testing.

  5. If patient is on thyroid therapy, discontinue treatment for 1 month before testing to determine baseline values.

  6. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Have patient resume normal activities.

  2. See Patient Care for Thyroid Testing.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Clinical Alert

  1. This test has nothing to do with the actual T3 blood level despite its name, which is sometimes confusingly abbreviated to the T3 test. It is emphasized that the T3 U and the true T3 are entirely different tests. The T3 U gives only an indirect measurement of overall binding.

  2. This test should be used only in conjunction with the T4 test to calculate the FTI.

  3. Some methods of determining T3 U have a direct relation with T4. Check the reference values of your laboratory.

Interfering Factors ⬆

  1. Decreased T3U levels occur in normal pregnancy and with drugs such as estrogens, antiovulatory drugs, methadone, and heparin.

  2. Increased T3U levels occur with drugs such as dicumarol, heparin, androgens, anabolic steroids, phenytoin, and large doses of salicylates.