Ronald Reagan UCLA Medical Center Clinical Laboratory UCLA Outreach Clinical Laboratory - Panorama City (BURL)
Performing Section
Chemistry
Availability
Ronald Reagan UCLA Medical Center Clinical Laboratory: Daily, 24 hours UCLA Outreach Clinical Laboratory - Panorama City (BURL): Monday through Friday 0600 - 0230, Saturday 1200-2030 (excluding holidays)
Turnaround Time
Ronald Reagan UCLA Medical Center Clinical Laboratory: 4 hours from receipt in the laboratory UCLA Outreach Clinical Laboratory - Panorama City (BURL): Routine: 24 hours; STAT: 4 hours from receipt in the laboratory
Free (unbound) thyroxine (FT4) is the biologically active portion of T4 and exists in equilibrium with the biologically inactive protein-bound T4 in the circulation. Its concentration is held constant by a complex and sensitive feedback control system involving the hypothalamus, anterior pituitary, and thyroid glands. This regulatory system is unaffected by changes in the concentration of protein-bound T4. Disorders of one of these glands may lose normal regulation of free T4 concentration in serum, resulting in hypothyroidism or hyperthyroidism. Abnormalities in serum protein, either their quantities or their affinity for T4, will elicit misleading elevated or low total T4 and free T4 index measurements in otherwise euthyroid individuals as in cases of TBG excess or deficiency, familial dysalbuminemia, and prealbumin abnormalities. However, free T4 levels in serum are high in hyperthyroidism but within the normal range in cases of hyperthyroxinemia of severe TBG excess and familial dysalbuminemic hyperthyroxinemia, are low in hypothyroidism, normal or increased in the hypothyroxinemias of nonthyroidal illness. Measurement of free T4 without interference from the concentration of the T4-binding proteins and their abnormalities is essential in differentiating euthyroid hyperthyroxinemia from hyperthyroidism and euthyroid hypothyroxinemia from hypothyroidism.
Patient Preparation
Whenever possible patient should stop taking supplements containing high concentrations of biotin (vitamin B7) for at least 12 hours prior to blood collection.
Limitations
High concentrations of biotin in patient's blood may lead to falsely increased free T4 results.
Specimen Type
Blood
Container
Light Green/Lithium Heparin Dark Green Microtainer Gold/SST Red Microtainer
Free T4 reference range based on age: 0 to 6 days: 0.9-2.5 ng/dL >6 days to 3 months: 0.9-2.2 ng/dL >3 months to 12 months: 0.9-2.0 ng/dL >1 year to 6 years: 1.0-1.8 ng/dL
Test Information
CPT Codes
84439
Synonyms
Free Thyroxine
FT4
T4, Free
Thyroxine, Free
Performing Laboratory / Facility
Ronald Reagan UCLA Medical Center Clinical Laboratory UCLA Outreach Clinical Laboratory - Panorama City (BURL)
Performing Section
Chemistry
Availability
Ronald Reagan UCLA Medical Center Clinical Laboratory: Daily, 24 hours UCLA Outreach Clinical Laboratory - Panorama City (BURL): Monday through Friday 0600 - 0230, Saturday 1200-2030 (excluding holidays)
Turnaround Time
Ronald Reagan UCLA Medical Center Clinical Laboratory: 4 hours from receipt in the laboratory UCLA Outreach Clinical Laboratory - Panorama City (BURL): Routine: 24 hours; STAT: 4 hours from receipt in the laboratory
Free (unbound) thyroxine (FT4) is the biologically active portion of T4 and exists in equilibrium with the biologically inactive protein-bound T4 in the circulation. Its concentration is held constant by a complex and sensitive feedback control system involving the hypothalamus, anterior pituitary, and thyroid glands. This regulatory system is unaffected by changes in the concentration of protein-bound T4. Disorders of one of these glands may lose normal regulation of free T4 concentration in serum, resulting in hypothyroidism or hyperthyroidism. Abnormalities in serum protein, either their quantities or their affinity for T4, will elicit misleading elevated or low total T4 and free T4 index measurements in otherwise euthyroid individuals as in cases of TBG excess or deficiency, familial dysalbuminemia, and prealbumin abnormalities. However, free T4 levels in serum are high in hyperthyroidism but within the normal range in cases of hyperthyroxinemia of severe TBG excess and familial dysalbuminemic hyperthyroxinemia, are low in hypothyroidism, normal or increased in the hypothyroxinemias of nonthyroidal illness. Measurement of free T4 without interference from the concentration of the T4-binding proteins and their abnormalities is essential in differentiating euthyroid hyperthyroxinemia from hyperthyroidism and euthyroid hypothyroxinemia from hypothyroidism.
Patient Preparation
Whenever possible patient should stop taking supplements containing high concentrations of biotin (vitamin B7) for at least 12 hours prior to blood collection.
Limitations
High concentrations of biotin in patient's blood may lead to falsely increased free T4 results.
Specimen Collection and Handling
Specimen Type
Blood
Container
Light Green/Lithium Heparin Dark Green Microtainer Gold/SST Red Microtainer
Free T4 reference range based on age: 0 to 6 days: 0.9-2.5 ng/dL >6 days to 3 months: 0.9-2.2 ng/dL >3 months to 12 months: 0.9-2.0 ng/dL >1 year to 6 years: 1.0-1.8 ng/dL
