Thyroxine, Total

Thyroxine, Total

Synonym/acronym: T4.

Common use

A first look at thyroid function and a tool to evaluate the effectiveness of therapeutic thyroid therapy.

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 × 17.1)
Cord blood6.6–17.5 mcg/dL113–299 nmol/L
1–30 days5.4–22.6 mcg/dL92–386 nmol/L
1 mo–23 mo5.4–16.6 mcg/dL92–284 nmol/L
2–6 yr5.3–15 mcg/dL91–256 nmol/L
7–11 yr5.7–14.1 mcg/dL98–241 nmol/L
12–19 yr4.7–14.6 mcg/dL80–250 nmol/L
Adult4.6–12 mcg/dL79–205 nmol/L
 Pregnant female5.5–16 mcg/dL94–274 nmol/L
Over 60 yr5–10.7 mcg/dL86–183 nmol/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 (see monograph titled “Thyroxine, Free”). Untreated deficiency of T4in newborns can result in untreatable, severe intellectual deficits and growth impairment. Neonatal screening for hypothyroidism is mandatory in all 50 states.

This procedure is contraindicated for

    N/A

Indications

  • Evaluate signs of hypothyroidism or hyperthyroidism and neonatal screening for congenital hypothyroidism (required in all 50 states)
  • Evaluate thyroid response to protein deficiency associated with severe illnesses
  • Monitor response to therapy for hypothyroidism or hyperthyroidism

Potential diagnosis

Increased in

  • Acute psychiatric illnesses (pathophysiology is unknown, although there is a relationship between thyroid hormone levels and certain types of mental illness)
  • Excessive intake of iodine (iodine is rapidly taken up by the body to form thyroxine)
  • Hepatitis (related to decreased production of TBG by damaged liver cells)
  • Hyperthyroidism (thyroxine is produced independently of stimulation by TSH)
  • Obesity
  • Thyrotoxicosis due to Graves’ disease (thyroxine is produced independently of stimulation by TSH)
  • Thyrotoxicosis factitia (laboratory tests do not distinguish between endogenous and exogenous sources)

Decreased in

    Decreased TBG (nephrotic syndrome, liver disease, gastrointestinal protein loss, malnutrition) Hypothyroidism (thyroid hormones are not produced in sufficient quantities regardless of TSH levels) Panhypopituitarism (dysfunctional pituitary gland does not secrete enough thyrotropin to stimulate the thyroid to produce thyroxine) Strenuous exercise

Critical findings

  • Hypothyroidism: Less than 2 mcg/dL (SI: Less than 34.2 nmol/L)
  • Hyperthyroidism: Greater than 20 mcg/dL (Greater than 342 nmol/L)
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • At levels less than 2 mcg/dL, the patient is at risk for myxedema coma. Signs and symptoms of severe hypothyroidism include hypothermia, hypotension, bradycardia, hypoventilation, lethargy, and coma. Possible interventions include airway support, hourly monitoring for neurological function and blood pressure, and administration of IV thyroid hormone.

  • At levels greater than 20 mcg/dL, the patient is at risk for thyroid storm. Signs and symptoms of severe hyperthyroidism include hyperthermia, diaphoresis, vomiting, dehydration, and shock. Possible interventions include supportive treatment for shock, fluid and electrolyte replacement for dehydration, and administration of antithyroid drugs (propylthiouracil and Lugol’s solution).

Interfering factors

  • Drugs that may increase T4 levels include amiodarone, amphetamines, corticosteroids, ether, fluorouracil, glucocorticoids, halofenate, insulin, iobenzamic acid, iopanoic acid, ipodate, levarterenol, levodopa, levothyroxine, opiates, oral contraceptives, phenothiazine, and prostaglandins.
  • Drugs, substances, and treatments that may decrease T4 levels include acetylsalicylic acid, aminoglutethimide, aminosalicylic acid, amiodarone, anabolic steroids, anticonvulsants, asparaginase, barbiturates, carbimazole, chlorpromazine, chlorpropamide, cholestyramine, clofibrate, cobalt, colestipol, corticotropin, cortisone, cotrimoxazole, cytostatic therapy, danazol, dehydroepiandrosterone, dexamethasone, diazepam, diazo dyes (e.g., Evans blue), dinitrophenol, ethionamide, fenclofenac, halofenate, hydroxyphenylpyruvic acid, interferon alfa-2b, iothiouracil, iron, isotretinoin, liothyronine, lithium, lovastatin, methimazole, methylthiouracil, mitotane, norethindrone, penicillamine, penicillin, phenylacetic acid derivatives, phenylbutazone, potassium iodide, propylthiouracil, reserpine, salicylate, sodium nitroprusside, stanozolol, sulfonylureas, tetrachlorothyronine, tolbutamide, and triiodothyronine (T3).

Nursing Implications and Procedure

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.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • 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.

Related Monographs

  • Related tests include albumin, antibodies antithyroglobulin, biopsy thyroid, copper, newborn screening, PTH, prealbumin, protein, RAIU, thyroglobulin, TBII, thyroid scan, TSH, TSI, free 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.
References in periodicals archive ?
318) measured the OP flame retardants tris (1, 3-dichloro-2-propyl) phosphate (TDCPP) and triphenyl phosphate (TPP) in house dust samples collected from 50 male infertility clinic patients and estimated associations with serum hormone concentrations (luteinizing hormone, follicle-stimulating hormone, estradiol, prolactin, free thyroxine, total triiodothyronine, and thyrotropin) and semen quality parameters (concentration, motility, and morphology).
Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: effects on plasma total thyroxine, total triiodothyronine, free thyroxine and growth hormone.