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.

Specimen

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

Normal findings

(Method: Immunoassay)

Age	Conventional Units	SI Units (Conventional Units × 0.0154)
0–3 days	2–7.9 pg/mL	0.03–0.12 pmol/L
4–30 days	2–5.2 pg/mL	0.03–0.08 pmol/L
1–23 mo	1.6–6.4 pg/mL	0.02–0.1 pmol/L
2–6 yr	2–6 pg/mL	0.03–0.09 pmol/L
7–17 yr	2.9–5.1 pg/mL	0.04–0.08 pmol/L
Adults and older adults	2.6–4.8 pg/mL	0.04–0.07 pmol/L
Pregnant women (4–9 mo gestation)	2–3.4 pg/mL	0.03–0.05 pmol/L

Description

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

N/A

Indications

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

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

(related to protein or iodine deficiency; iodine is needed for thyroid hormone synthesis and proteins are needed for transport)

Critical findings

N/A

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

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 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.