Thyroxine-binding globulin (TBG) may be measured directly by radioimmunoassay. Estrogens elevate serum TBG levels; thus, women who are pregnant, who are receiving estrogen therapy or oral contraceptives, or who have estrogen-secreting tumors have higher TBG levels.

• Estrogens elevate serum TBG levels and, thus, women who are pregnant, who are receiving estrogen therapy or oral contraceptives, or who have estrogen-secreting tumors have higher TBG levels.
• Androgens and corticosteriods decrease serum TBG levels.

• Signs and symptoms of hypothyroidism or hyperthyroidism in conditions associated with altered TBG levels (e.g., pregnancy), to differentiate true thyroid disorders from problems related to altered TBG levels
• Diagnosis of hereditary abnormality of globulin synthesis, indicated by decreased levels

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 may alter TBG levels 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 collection of a peripheral blood sample.

• Resume any medications withheld before the test.