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Introduction ⬇

Almost all of the thyroid hormones in the blood are protein bound: albumin, thyroid-binding PAB, and, most importantly, TBG. Variations in TBG levels have a major effect on bound and free (metabolically active) forms of T4 and T3. Before considering this test, TSH, FTI, and total T4 should be measured.

The TBG measurement is useful to distinguish between hyperthyroidism causing high T4 levels and euthyroidism with increased binding by TBG, increased T4, and normal levels of free hormones; to identify hereditary deficiency or increase of TBG; and to work up thyroid disease in hypothyroid populations, when the mean TBG concentration is significantly higher than the mean level in normal thyroid populations. In hyperthyroid populations, the mean TBG level concentration is lower than the mean level in normal thyroid populations.

Normal Findings ⬆ ⬇

Infants: 3–6 mg/dL or 30–60 mg/L

Men: 1.2–2.5 mg/dL or 12–25 mg/L

Women: 1.4–3.0 mg/dL or 14–30 mg/L

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood specimen (red-topped tube). 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 ⬆ ⬇

  1. TBG is increased in the following conditions:

    1. Genetically determined high TBG

    2. Hypothyroidism (some cases)

    3. Infectious hepatitis

    4. Acute intermittent porphyria

    5. Estrogen-producing tumors (endogenous or exogenous)

    6. Late-stage HIV infections

  2. TBG is decreased in the following conditions:

    1. Genetic deficiency of TBG

    2. Nephrotic syndrome

    3. Major illness, surgical stress

    4. Ovarian hypofunction

    5. Acromegaly

    6. Chronic liver disease

    7. Marked hypoproteinemia, malnutrition

Interventions ⬆ ⬇

Pretest Patient Care

  1. See Patient Care for Thyroid Testing.

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

Posttest Patient Care

  1. See Patient Care for Thyroid Testing.

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

Interfering Factors ⬆

  1. Many drugs increase (e.g., estrogens, oral contraceptives) or decrease (e.g., nicotinic acid, phenytoin, steroids) values (see Appendix E).

  2. Neonates have higher values.

  3. Recently administered radioisotopes affect results.

  4. Pregnancy increases levels.

  5. Prolonged heroin use or methadone increases levels.