Thyroxine-Binding Globulin

Synonym/Acronym

TBG.

Rationale

To evaluate thyroid hormone levels related to deficiency or excess, to assist in diagnosing disorders such as hyperthyroidism and hypothyroidism.

Patient Preparation

There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings

Method: Chemiluminescent Immunoassay.

Conventional Units		SI Units (Conventional Units × 1)
13–39 mcg/mL		13–39 mg/L

Critical Findings and Potential Interventions ⬆ ⬇

Overview ⬆ ⬇

Study type: Blood collected in a gold-, red-, or red/gray-top tube; related body system: Endocrine system.

Thyroxine-binding globulin (TBG) is the predominant transport protein for the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃). T₄-binding prealbumin and T₄-binding albumin are the other transport proteins. Conditions that affect TBG levels and binding capacity also affect free T₃ and free T₄ levels.

Indications ⬆ ⬇

Differentiate elevated T₄ due to hyperthyroidism from increased TBG binding in euthyroid patients.
Evaluate hypothyroid patients.
Identify deficiency or excess of TBG due to hereditary abnormality.

Interfering Factors ⬆ ⬇

Factors That May Alter the Results of the Study

Drugs and other substances that may increase TBG levels include estrogens, oral contraceptives, perphenazine, and tamoxifen.
Drugs and other substances that may decrease TBG levels include anabolic steroids, asparaginase, corticosteroids, danazol, phenytoin, and propranolol.

Potential Medical Diagnosis: Clinical Significance of Results ⬆ ⬇

Increased In

Acute intermittent porphyria (pathophysiology is unclear)
Estrogen therapy(TBG is increased in the presence of exogenous or endogenous estrogens)
High TBG levels (rare, inherited)
Hyperthyroidism (related to increased levels of total thyroxine available for binding)
Infectious hepatitis and other liver diseases (pathophysiology is unclear)
Neonates
Pregnancy (TBG is increased in the presence of exogenous or endogenous estrogens)

Decreased In

Acromegaly
Chronic hepatic disease (related to general decrease in protein synthesis)
Low TBG levels (inherited or acquired)
Major illness (related to general decrease in protein synthesis)
Marked hypoproteinemia, malnutrition (related to general decrease in protein synthesis)
Nephrotic syndrome(related to general increase in protein loss)
Ovarian hypofunction (TBG is decreased in the absence of estrogens)
Surgical stress (related to general decrease in protein synthesis)
Testosterone-producing tumors(TBG is decreased in the presence of testosterone)

Nursing Implications, Nursing Process, Clinical Judgement ⬆

Before the Study: Planning and Implementation

Teaching the Patient What to Expect

Discuss how this test can assist in assessing thyroid gland function.
Explain that a blood sample is needed for the test.

After the Study: Implementation & Evaluation Potential Nursing Actions

Treatment Considerations

Answer any questions or address any concerns voiced by the patient or family.
Discuss symptoms of hypothyroidism and hyperthyroidism as applies to the clinical situation.

Hypothyroidism

Symptoms of hypothyroidism in adults include cold sensitivity, fatigue, weight gain, weakness, constipation, thinning hair, hoarseness, muscle weakness, memory impairment, depression, dry skin, muscular aches, and joint pain.
Symptoms of hypothyroidism in children and teens include delays in growth, puberty, development of permanent teeth, constipation, weight gain without increased dietary intake, swollen or puffy appearance, and poor mental development.
Symptoms of hypothyroidism in infants include hoarse crying, large protruding tongue, umbilical hernia, constipation, poor muscle tone, difficulty breathing, sleepiness.

Hyperthyroidism

Symptoms of hyperthyroidism in adults include tachycardia, arrhythmia, palpitations, tremors, sweating, heat sensitivity, weight loss, anxiety, fatigue, insomnia, thinning nails and hair, frequent bowel movements, tremors, sweating, nervousness, anxiety, irritability.
Symptoms of hyperthyroidism in children include tachycardia, sweating, sleeping problems, increased appetite with weight loss, wide-eyed stare, bulging eyes, loose stool, trembling hands, difficulty concentrating.
Symptoms of hyperthyroidism in infants include tachycardia, rapid breathing, irritability, failure to thrive, diarrhea, increased appetite without weight gain, bulging eyes, skull bones close early.

Clinical Judgement

Consider how to address parental concerns (cultural, religious) to facilitate treatment for the pediatric population.

Follow-Up and Desired Outcomes

Understands that depending on the results of this study, additional testing may be performed to monitor disease progression and determine the need for a change in therapy.