Thyroxine (T₄) is measured by competitive protein binding or by radioimmunoassay. In competitive protein binding, the affinity between T₄ and TBG is exploited. Reagent TBG fully saturated with radiolabeled T₄ is incubated with T₄ extracted from the client's serum. The T₄ from the test serum displaces the radiolabeled T₄ in the amount present. This procedure is known as T4 by displacement (T₄ D), T4 by competitive binding (T₄ CPB), and T4 Murphy-Pattee (T₄ MP). T4 measured by radioimmunoassay (T₄ RIA) is the preferred method to measure T₄ because it is not affected by circulating iodinated substances.

Most T₄ (99.97 percent) in the serum is bound to TBG. The remainder circulates as unbound ("free") T₄ (FT₄) and is responsible for all of the physiological activity of thyroxine. Because FT₄ is not dependent on normal levels of TBG, as is the case with total serum thyroxine, FT₄ levels are considered the most accurate indicator of thyroxine and its thyrometabolic activity. It is difficult, however, to measure FT₄ directly because quantities are so small and the interference from bound T₄ is great. Free hormone levels are, therefore, usually calculated by multiplying the values for total T₄ by the T₃ uptake ratio. The result is expressed as the free thyroxine index (FT₄ I). The free hormone index varies directly with the amount of circulating hormone and inversely with the amount of unsaturated TBG present in the serum.⁴⁹

Note: Values may vary according to the laboratory performing the test. Results should be evaluated in relation to other tests of thyroid function.

• Results of T₄ D may be altered by circulating iodinated substances; T₄ RIA is not similarly affected.
• Pregnancy, estrogen therapy, or estrogen-secreting tumors may produce elevated T₄ levels.
• Ingestion of thyroxine will elevate T₄ levels.
• Heroin and methadone may produce elevated T₄ levels.
• Androgens, glucocorticoids, heparin, salicylates, phenytoin anticonvulsants, sulfonamides, and antithyroid drugs such as propylthiouracil may lead to decreased T₄ levels.

• Signs of hypothyroidism, hyperthyroidism, or neonatal screening for congenital hypothyroidism (required in many states), or hypothyroidism or hyperthyroidism combined with neonatal screening:
  • Decreased T₄ and FT₄ levels indicate hypothyroid states and also may be seen in early thyroiditis.
  • Elevated T₄ and FT₄ levels indicate hyperthyroid states.
  • Normal T₄ and FT₄ levels in clients with signs of hyperthyroidism may indicate T₃ thyrotoxicosis.
  • Normal FT₄ levels are seen in pregnancy, whereas T₄ and TBG are usually elevated.
• Monitoring of response to therapy for hypothyroidism or hyperthyroidism:
  • Elevated T₄ and FT₄ levels indicate response to treatment for hypothyroidism.
  • Decreased T₄ and FT₄ levels indicate response to treatment for hyperthyroidism.
• Evaluation of thyroid response to protein deficiency associated with severe illnesses (e.g., metastatic cancer, liver disease, renal disease, diabetes mellitus, cardiovascular disorders, burns, and trauma):
  • T₄ is decreased in such disorders because of a deficiency of TBG, a protein.
  • FT₄ index is normal, if thyroid function is normal, because FT₄ index is not dependent on TBG 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 usually recommended that thyroid medications be withheld for 1 month before the test and that other drugs that may alter thyroxine levels be withheld for at least 24 hours before the study. This practice should be confirmed, however, with the person ordering the test.
• For infants, explain to the parent(s) the purpose of the test and that it may require repetition in 3 to 6 weeks because of normal changes in infant thyroid hormone levels.

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.

• For neonatal screening, the sample is obtained by heelstick. A multiple neonatal screening kit is usually used; the directions provided with the kit must be followed carefully.

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.
• Increased levels: Note and report increased levels and relation to other thyroid tests and procedures performed. Assess for signs and symptoms of hyperthyroidism such as tachycardia, increased appetite, diaphoresis, elevated temperature, exophthalmos, weight loss, insomnia, hyperactivity, or inability to handle stress. Prepare for possible radionuclide therapy or surgical intervention. Administer ordered medications to reduce levels. Instruct in adequate fluid and nutritional dietary intake and eye care for exophthalmos. Instruct client to avoid stressful situations.
• Decreased levels: Note and report decreased levels and relation to other thyroid tests and procedures.. Assess for cold intolerance, weight gain, skin changes, constipation, lethargy, or fatigue. Administer ordered replacement therapy. Instruct client in appropriate fluid and low caloric nutritional dietary intake, long-term thyroid medication regimen, control of environment for relaxation and warmth, and care of skin and hair. Instruct client to avoid sedatives to promote sleep.