Thyroxine, Free

Thyroxine, Free

Synonym/acronym: Free T4, Free4.

Common use

A complementary laboratory test in evaluating thyroid hormone levels related to deficiency or excess to assist in diagnosing hyperthyroidism and hypothyroidism.

Specimen

Serum (1 mL) collected in a gold-, red-, or red/gray-top tube. Plasma (1 mL) collected in a green-top (heparin) tube is also acceptable.

Normal findings

(Method: Immunoassay)
AgeConventional UnitsSI Units (Conventional Units × 12.9)
Newborn0.8–2.8 ng/dL10–36 pmol/L
1–12 mo0.8–2 ng/dL10–26 pmol/L
1–18 yr0.8–1.7 ng/dL10–22 pmol/L
Adult–older adult0.8–1.5 ng/dL10–19 pmol/L
Pregnancy (1st trimester)0.9–1.4 ng/dL12–18 pmol/L
Pregnancy (2nd trimester)0.7–1.3 ng/dL 9–17 pmol/L

Description

Thyroxine (T4) is a hormone produced and secreted by the thyroid gland. Most T4 in the serum (99.97%) is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin. The remainder (0.03%) circulates as unbound or free T4, which is the physiologically active form. Levels of free T4 are proportional to levels of total T4. The advantage of measuring free T4 instead of total T4 is that, unlike total T4 measurements, free T4 levels are not affected by fluctuations in TBG levels; as a result, free T4 levels are considered the most accurate indicator of T4 and its thyrometabolic activity. Free T4 measurements are useful in evaluating thyroid disease when thyroid-stimulating hormone (TSH) levels alone provide insufficient information. Free T4 and TSH levels are inversely proportional. Measurement of free T4 is also recommended during treatment for hyperthyroidism until symptoms have abated and levels have decreased into the normal range.

This procedure is contraindicated for

    N/A

Indications

  • Evaluate signs of hypothyroidism or hyperthyroidism
  • Monitor response to therapy for hypothyroidism or hyperthyroidism

Potential diagnosis

Increased in

  • Hyperthyroidism (thyroxine is produced independently of stimulation by TSH)
  • Hypothyroidism treated with T4(laboratory tests do not distinguish between endogenous and exogenous sources)

Decreased in

    Hypothyroidism (thyroid hormones are not produced in sufficient quantities regardless of TSH levels) Pregnancy (late)

Critical findings

    N/A

Interfering factors

  • Drugs that may increase free T4 levels include acetylsalicylic acid, amiodarone, halofenate, heparin, iopanoic acid, levothyroxine, methimazole, and radiographic agents.
  • Drugs that may decrease free T4 levels include amiodarone, anabolic steroids, asparaginase, methadone, methimazole, oral contraceptives, and phenylbutazone.

Nursing Implications and Procedure

Potential nursing problems

ProblemSigns & SymptomsInterventions
Decreased cardiac output (Related to a deficit of thyroid hormone)Bradycardia; lethargy; hypotension; decreased thyroid hormone levels; fatigue; activity intolerance; poor peripheral perfusion; cool skin; shortness of breathAssess and trend vital signs and blood pressure; monitor and trend thyroid laboratory studies (TSH, T3, T4, RAIU); assess cardiac status indicators—peripheral pulses, skin color, skin temperature, dry scaly skin); assess for periorbital edema; administer prescribed thyroid hormone replacement medication; facilitate measures to improve patient warmth (blankets, warm clothing and liquids, warmer environment); pace activity and schedule rest periods to manage fatigue; use pulse oximetry to monitor oxygen saturation; assess respiratory status checking for crackles and increased respiratory rate; monitor for fluid overload
Altered thought processes (Related to decreased cardiac output and impaired cerebral perfusion secondary to a deficit of thyroid hormone)Altered memory; mental impairment; decreased concentration; depression; inaccurate environmental perception; inappropriate thinking; memory deficitsMinimize apprehension and dread; collaborate with the health-care provider (HCP) to manage medical problem associated with decreased cerebral perfusion; promote comprehension and understanding of current events; provide a modified environment that promotes safety; monitor the ability to provide self-care (activities of daily living); monitor injury risk (violence, fall risk, self-harm risk); administer prescribed thyroid hormone replacement medication
Body image (Related to the presence of goiter and exophthalmos)Eye bulging from exophthalmos; enlarged neck from goiter; distress and refusal to talk about changed appearance; negative verbalization about changes in appearance; uses clothing or other devices to hide changed appearance (scarves, high necks, sunglasses)Assess the patient's perception of physical changes; note the frequency of negative comments about changed physical state; assist in the identification of positive coping strategies to address changed physical appearance
Elevated temperature (Related to increased basal metabolic rate)Elevated temperature; flushed; warm skin; diaphoresisAssess the patient's temperature frequently; monitor for emotionally labile events that could precipitate a thyroid storm or crisis and precipitate an elevation in temperature; ensure the patient's immediate environment remains cool; encourage the use of light bedding and lightweight clothing to prevent overheating; increase fluid intake to offset insensible fluid loss; encourage bathing with tepid water for comfort and promotion of cooling; administer prescribed antithyroid therapy

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in assessing thyroid gland function.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s endocrine system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

  • Patient Education

    • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP.
    • Recognize anxiety related to test results, and answer any questions or address any concerns voiced by the patient or family.
    • Teach the family and patient about the relationship between the development of goiter and exophthalmos and hyperthyroidism.

  • Expected Patient Outcomes

    • Knowledge
    • States understanding of symptoms that may indicate a thyroid storm
    • Identifies symptoms that would indicate hypothyroidism
    • Skills
    • Demonstrates proficiency in selecting clothing that will assist in remaining cool and prevent overheating
    • Demonstrates proficiency in the self-administration of thyroid or anti-thyroid medication correctly as prescribed
    • Attitude
    • Displays acceptance of changed appearance and refrains from negative self-comments
    • Complies with recommended medication regime

Related Monographs

  • Related tests include albumin, antibodies antithyroglobulin, biopsy thyroid, copper, PTH, prealbumin, protein, RAIU, thyroglobulin, TBII, thyroid scan, TSH, TSI, T4, T3, free T3, and US thyroid.
  • Refer to the Endocrine System table at the end of the book for related tests by body system.