Triiodothyronine, Free

Triiodothyronine, Free

Synonym/acronym: Free T3, FT3.

Common use

A complementary adjunct to evaluate thyroid hormone levels primarily related to hyperthyroidism and to assess causes of hypothyroidism.


Serum (1 mL) collected in a gold-, red-, or red/gray-top tube.

Normal findings

(Method: Immunoassay)
AgeConventional UnitsSI Units (Conventional Units × 0.0154)
0–3 days2–7.9 pg/mL0.03–0.12 pmol/L
4–30 days2–5.2 pg/mL0.03–0.08 pmol/L
1–23 mo1.6–6.4 pg/mL0.02–0.1 pmol/L
2–6 yr2–6 pg/mL0.03–0.09 pmol/L
7–17 yr2.9–5.1 pg/mL0.04–0.08 pmol/L
Adults and older adults2.6–4.8 pg/mL0.04–0.07 pmol/L
Pregnant women (4–9 mo gestation)2–3.4 pg/mL0.03–0.05 pmol/L


Unlike the thyroid hormone thyroxine (T4), most T3 is converted enzymatically from T4 in the tissues rather than being produced directly by the thyroid gland (see monograph titled “Thyroxine, Total”). Approximately one-third of T4 is converted to T3. Most T3 in the serum (99.97%) is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin. The remainder (0.03%) circulates as unbound or free T3, which is the physiologically active form. Levels of free T3 are proportional to levels of total T3. The advantage of measuring free T3 instead of total T3 is that, unlike total T3 measurements, free T3 levels are not affected by fluctuations in TBG levels. T3 is four to five times more biologically potent than T4. This hormone, along with T4, is responsible for maintaining a euthyroid state. Free T3 measurements are rarely required, but they are indicated in the diagnosis of T3 toxicosis and when certain drugs are being administered that interfere with the conversion of T4 to T3.

This procedure is contraindicated for



  • Adjunctive aid to thyroid-stimulating hormone (TSH) and free T4 assessment
  • Assist in the diagnosis of T3 toxicosis

Potential diagnosis

Increased in

  • High altitude
  • Hyperthyroidism (triiodothyronine is produced independently of stimulation by TSH)
  • T3 toxicosis

Decreased in

    Hypothyroidism (thyroid hormones are not produced in sufficient quantities regardless of TSH levels) Malnutrition (related to protein or iodine deficiency; iodine is needed for thyroid hormone synthesis and proteins are needed for transport) Nonthyroidal chronic diseases Pregnancy (late)

Critical findings


Interfering factors

  • Drugs that may increase free T3 include acetylsalicylic acid, amiodarone, and levothyroxine.
  • Drugs that may decrease free T3 include amiodarone, methimazole, phenytoin, propranolol, and radiographic agents.

Nursing Implications and Procedure


  • 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.


  • 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.


  • Inform the patient that a report of the results will be made available to the requesting health-care provider (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, PTH, prealbumin, protein, RAIU, thyroglobulin, TBII, thyroid scan, TSH, TSI, T4, free T4, T3, and US thyroid.
  • Refer to the Endocrine System table at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
References in periodicals archive ?
Values are expressed as mean [+ or -] standard deviation, independent samples t-test was used for free triiodothyronine, free thyroxine, thyroid stimulating hormone and reverse triiodothyronine.
Before operation, the patient's thyroid was normal without abnormal findings of thyroid ultrasonography, thyroid function tests (triiodothyronine, thyroxine, free triiodothyronine, free thyroxine, and hypersensitive thyrotropin), thyroid globulin, and relative thyroid antibodies (thyroglobulin antibody, thyroperoxidase antibody, TRAb).
Serum insulin-like growth factor-1 and insulin-like growth factor-binding protein 3 levels were low, but free triiodothyronine, free thyroxine, and adrenocorticotropic hormone levels were almost normal.
Clinical and paraclinical findings (free triiodothyronine, free thyroxine and thyroid stimulating hormone) were documented for all patients.
Measurement of serum total thyroxine, triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs.
Serum thyroid-stimulating hormone, free triiodothyronine, free thyroxine, ACTH, and cortisol values were within the reference intervals for healthy individuals in all obese patients.
Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: effects on plasma total thyroxine, total triiodothyronine, free thyroxine and growth hormone.
Among other changes in the leptin-treated subjects but not the controls were increases in free triiodothyronine, free thyroxine, and insulin-like growth factor-I.