Free Thyroxine (FT₄)

FT₄ comprises a small fraction of total T₄. The FT₄ is unbound to protein and available to the tissues, and it is the metabolically active form of this hormone. This fraction constitutes about 5% of the circulatory T₄.

FT₄ has diagnostic value in situations in which total hormone levels do not correlate with the thyrometabolic state and there is suspected abnormality in thyroxine-binding globulin (TBG) levels. It provides a more accurate picture of the thyroid status in persons with abnormal TBG levels in pregnancy and in those who are receiving estrogens, androgens, phenytoin, or salicylates.

0.7–2.0 ng/dL or 10–26 pmol/L

For patients taking levothyroxine (Synthroid), up to 5.0 ng/dL or 64 pmol/L

Procedures

1. Obtain a 5-mL venous blood sample. Accurate results can be obtained with as little as 0.5 mL of blood in pediatric patients. Serum is needed for this test.

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.

1. Increased FT₄levels are associated with the following conditions:

   1. Graves disease (hyperthyroidism)

   2. Hypothyroidism treated with T₄

   3. Euthyroid sick syndrome

2. Decreased FT₄levels are associated with the following conditions:

   1. Primary hypothyroidism

   2. Secondary hypothyroidism (pituitary)

   3. Tertiary hypothyroidism (hypothalamic)

   4. Hypothyroidism treated with T₃

Pretest Patient Care

1. See Patient Care for Thyroid Testing. The same protocols prevail for FT₄.

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

1. Values are increased in infants at birth and rise even higher after 2–3 days of life.

2. Many drugs affect test outcomes (see Appendix E).

3. Heparin causes falsely elevated FT₄ values.

4. Levels can fluctuate in patients with severe or chronic illness.

5. Levels fluctuate in pregnancy (low in late pregnancy).