The T₃ uptake (RT₃ U) test evaluates the quantity of TBG in the serum and the quantity of thyroxine (T₄) bound to it. In the T₃ uptake procedure, a known amount of resin containing radiolabeled T₃ is added to a sample of the client's serum. Normally, TBG in the serum is not fully saturated with thyroid hormones; the saturation level varies in relation to the amounts of TBG and thyroid hormones present. In the T₃ uptake test, the radiolabeled T₃ binds with available TBG sites. Results of the test are determined by measuring the percentage of labeled T₃ that remains bound to the resin after all available sites on TBG have been filled. Note that the percentage of T₃ bound to the resin is inversely proportional to the percentage of TBG saturation in the serum.

Results of the T₃ uptake test are evaluated in relation to serum levels of total T₄ and T₃ and also are used in calculating FT₃ and FT₄ indices. For these calculations, the T₃ uptake ratio (RT₃ UR) is used, a ratio obtained by dividing the client's RT₃ U level by the RT₃ U level determined from a pool of normal serum

• Drugs that alter TBG levels or that compete for TBG-binding sites may affect test results.
• Estrogens may lead to increased TBG levels.
• Androgens and glucocorticoids may lead to decreased TBG levels.
• Salicylates and phenytoin anticonvulsants compete with T₄ for TBG-binding sites.
• Results may vary during pregnancy when TBG levels are usually elevated.

• Signs of hypothyroidism or hyperthyroidism:
  • Decreased levels (indicating a low percentage of radiolabeled T₃ remaining) indicate low serum T₄ levels and hypothyroidism.
  • Elevated levels (indicating a high percentage of radiolabeled T₃ remaining) indicate high serum T₄ levels and hyperthyroidism.
• Known or suspected problems associated with altered TBG levels (e.g., hereditary abnormality of TBG synthesis, drug therapy, pregnancy, and disorders associated with decreased serum proteins):
  • Elevated levels may indicate low TBG levels.
  • Decreased levels may indicate elevated TBG levels.
• Monitoring for response to therapy with drugs that compete with T₄ for TBG-binding sites:
  • Elevated levels may indicate that TBG-binding sites are saturated with competing drugs.
• Calculation of free T₃ and T₄ indices

Nursing Care Before the Procedure

Client preparation is essentially the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

• It is recommended that drugs that alter TBG levels or compete for TBG-binding sites be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any medications withheld before the test.