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Introduction ⬇

Thyroxine (T4), Total

Thyroxine is the thyroid hormone with four atoms of iodine; hence, it is called T4. The combination of the serum T4 and T3 uptake as an assessment of TBG helps to determine whether an abnormal T4 value is due to alterations in serum TBG or to changes in thyroid hormone levels. Deviations of both tests in the same direction usually indicate that an abnormal T4 level is due to abnormalities in thyroid hormone. Deviations of the two tests in opposite directions provide evidence that an abnormal T4 may relate to alterations in TBG.

T4, one of the thyroid function panel tests, is a direct measurement of the concentration of T4 in the blood serum. Total T4 level is a good index of thyroid function when the TBG is normal. The increase in TBG levels normally seen in pregnancy and with estrogen therapy increases total T4 levels. The decrease of TBG levels in persons receiving anabolic steroids, in chronic liver disease, and in nephroses decreases the total T4 value. This test is commonly done to rule out hyperthyroidism and hypothyroidism. The T4 test also can be used as a guide in establishing maintenance doses of thyroid hormone in the treatment of hypothyroidism. In addition, it can be used in hyperthyroidism to follow the results achieved with antithyroid drug administration.

Normal Findings ⬆ ⬇

Adults: 5.4–11.5 μg/dL or 57–148 nmol/L

Children: 6.4–13.3 μg/dL or 83–172 nmol/L

Neonates: 11.8–22.6 μg/dL or 152–292 nmol/L

If testing is done by radioimmunoassay, it is reported as T4 RIA.

Clinical Alert

Critical Values>20 g/dL or >258 nmol/L: Thyroid storm is possible.2.0 g/dL or 26 nmol/L: Myxedema coma is possible.

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample. Serum is used. If the patient is already receiving thyroid treatment, it must be discontinued 1 month before the test.

  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 ⬆ ⬇

  1. Increased T4values are found in the following conditions:

    1. Hyperthyroidism (Graves disease, goiter)

    2. Clinical status that increases TBG

    3. Thyrotoxicosis factitia

    4. Acute thyroiditis

    5. Hepatitis, liver disease

    6. Lymphoma

  2. Decreased T4values are found in the following conditions:

    1. Hypothyroidism

    2. Disorders of decreased TBG

    3. Hypoproteinemia

    4. Treatment with T3

    5. Nephrotic syndrome

Clinical Alert

T4 values are higher in neonates due to elevated TBG. Values rise abruptly in the first few hours after birth and decline gradually until the age of 5 years

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure. T4 is usually the first test used in the diagnosis of hypothyroidism or hyperthyroidism, along with the TSH.

  2. Have patient avoid strenuous exercise.

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

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

  5. 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.

Interfering Factors ⬆

  1. Total T4 levels increase during the second or third month of pregnancy as a result of increased estrogen production. Normal range: 5.5–16.0 μg/dL or 71–206 nmol/L.

  2. Total T4 levels increase with the use of drugs such as estrogens, heroin, and methadone and excess iodine (see Appendix E).

  3. Contrast agents used for x-rays and other diagnostic procedures affect results.

  4. Values are decreased with salicylates, anticonvulsants, and steroids.