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Introduction ⬇

Free Triiodothyronine (FT3)

This is one of the determinations used to evaluate thyroid function and measure that fraction of the circulatory T3 that exists in the free form in the blood unbound to protein. Free T3 (FT3) is done to rule out T3 toxicosis, to evaluate thyroid replacement therapy, and to clarify protein-binding abnormalities.

Normal Findings ⬆ ⬇

Adults: 260–480 pg/dL or 4.0–7.4 pmol/L

Procedure ⬆ ⬇

  1. Obtain a 5-mL venous blood sample.

  2. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

Clinical Implications ⬆ ⬇

  1. Increased FT3values are associated with the following conditions:

    1. Hyperthyroidism

    2. T3 toxicosis

    3. Peripheral resistance syndrome

  2. Decreased FT3values are associated with the following conditions:

    1. Hypothyroidism (primary and secondary)

    2. Third trimester of pregnancy

Interventions ⬆ ⬇

Pretest Patient Care

  1. See Patient Care for Thyroid Testing. The same protocols prevail for FT3.

  2. Follow Chapter 1 guidelines for safe, effective, informed pretest care.

Posttest Patient Care

  1. See Patient Care for Thyroid Testing. The same protocols prevail in FT3 testing.

  2. Follow Chapter 1 guidelines for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Recently administered radioisotopes and some drugs (see Appendix E)

  2. High altitude: FT3 levels are higher