FREE T4 (THYROXINE)

Order Code

---

2823

Preferred Specimen

---

Collect 2 mL of serum. Allow the SST tube to clot upright for at least 30 minutes, then centrifuge the sample within 2 hours of collection. Refrigerate the specimen.

ContainerType

---

Serum separator tube

Alternate Specimen Requirements

---

A 2 mL serum sample collected in a plain red top tube is acceptable. Allow the sample to clot in an upright position for at least 60 minutes, then centrifuge and transfer the serum to a plastic transport tube within 2 hours. Clearly label the tube as serum from a plain red top tube. Refrigerate.

Minimum Volume

---

Adult: 1 mL serum
Pediatric: 0.2 mL serum (does not allow for repeat or
additional testing).

Transport Temperature

---

Refrigerated

Expected Turnaround Time

---

3 days

Specimen Stability

---

5 days room temperature; 1 week refrigerated; 1 month frozen. Allow only one freeze/thaw cycle

Methodology

---

Roche COBAS Electrochemiluminescence Immunoassay (ECLIA) this method has been standardized against the elecsys ft4 method. The elecsys ft4 assay is traceable to the enzymun- test ft4 which has been standardized using equilibrium dialysis.

Overview

---

Thyroxine, commonly referred to as T4 because it contains four iodine atoms, is one of the two primary hormones secreted by the thyroid gland, the other being triiodothyronine (T3). The secretion of T4 from the thyroid is stimulated by thyroid stimulating hormone (TSH) produced by the pituitary gland. In peripheral tissues like the liver and brain, T4 is converted into the more active T3. Pituitary secretion of TSH is regulated by feedback from circulating free T4 and T3 levels—low levels of these hormones increase TSH secretion, while high levels suppress it.

Total T4 measures both the hormone bound to proteins and the free fraction; however, only the free T4 is biologically active. At any time, the majority of circulating thyroxine (>99.9%) is bound to transport proteins such as thyroxine binding globulin (TBG), prealbumin, and albumin, with only a small percentage (0.03% to 0.04%) existing as free hormone. Changes in these carrier proteins influence total T4 levels but have minimal effect on free T4 concentrations.

Due to the influence of binding protein variations on total T4 levels in individuals who are otherwise euthyroid, reliance on total T4 testing has declined. Instead, the third-generation sensitive TSH assay (sTSH), alone or combined with a free T4 measurement, is preferred for thyroid function screening.

Clinical Significance

---

• Diagnose thyroid disease: FT₄ is preferred over total T₄; predominantly a second-line test after TSH to evaluate TSH changes
• May become first-line test in select conditions where TSH is diagnostically unreliable (eg, early phase of treating hypo- or hyperthyroidism, when hypothalamic-pituitary dysfunction is suspected to be present, or when patient is taking certain drugs such as glucocorticoids that affect TSH secretion)
• Assess severity of hypothyroidism (FT₄ decreased) and hyperthyroidism (FT₄ increased)
• Monitor therapy (replacement or suppressive)
• Evaluate hypothalamic-pituitary dysfunction (eg, central hypothyroidism)
• Indicator of thyroid function when TBG is normal, patient is not pregnant, or nonthyroidal illness in not present
• Diagnose thyroid disease, although FT₄measurement is preferred over total T₄
• Assess severity of hypothyroidism (T₄ decreased) and hyperthyroidism (T₄ increased)
• Monitor therapy (replacement or suppressive)
• Total T₄ measurement may be useful when patient is on heparin therapy as free fatty acids (generated during therapy) displace T₄ from albumin yielding a falsely high FT₄ result (Dufour, 2007; Gurnell, 2011).
• Used in calculation of free thyroxine index (FTI) (FTI = Total T₄ x T₃ uptake); originally considered to be a more accurate indicator of thyroid status in presence of abnormal concentrations of plasma proteins (TBG, albumin, prealbumin); however FTI has been replaced by FT₄ measurement.

Additional Information

---

Free T4 is a more accurate reflection of thyroid status than total T4 because it is less impacted by fluctuations in thyroxine-binding proteins. Measurement of free T4 is especially useful when abnormalities in binding globulins are suspected, or when total T4 and TSH results are inconclusive or inconsistent with the clinical picture. Typically, free T4 levels remain normal in euthyroid individuals with elevated thyroxine-binding globulin.

Thyroid disease screening is common in clinical practice due to the nonspecific nature of symptoms, its role in evaluating secondary conditions, and the assessment of thyroid nodules or goiters. Occasionally, sensitive TSH results may show mild abnormalities (either high or low) while free T4 remains within the normal range, suggesting “subclinical” thyroid dysfunction. In these cases, testing for thyroid autoantibodies may help identify patients at higher risk of progressing to overt thyroid disease. Additionally, assessment of total T3 is recommended to exclude T3 thyrotoxicosis (see Triiodothyronine, Total, Serum). Patients with subclinical thyroid profiles require periodic monitoring for potential progression. The presence of a goiter increases the likelihood of developing overt thyroid disease.

Interpretative Information

---

Decreased:

• Primary hypothyroidism (eg, Hashimoto thyroiditis, endemic goiter)
• Status postsurgery, radioactive iodine (RAI), or antithyroid medication (propylthiouracil, methimazole)
• Central hypothyroidism (eg, postpartum pituitary necrosis, tumors)
• Recovering subacute thyroiditis
• Conditions that reduce total T₄ (but not FT₄ as significantly) due to decreased proteins (TBG, prealbumin, albumin) such as malnutrition, nephrotic syndrome, GI protein loss, and chronic liver disease
• Drugs (eg, phenytoin, carbamazepine, anabolic steroids, interferon, interleukin-2, lithium)
• Significant nonthyroidal illness (NTI), also known as euthyroid sick syndrome − total T₄ is typically affected more than FT₄

Increased:

• Hyperthyroidism (eg, Graves disease, toxic multinodular or uninodular goiter, iodine-induced hyperthyroidism)
• Early Hashimoto thyroiditis
• First stage of (painful) subacute thyroiditis
• Thyrotoxicosis factitia
• Peripheral resistance to thyroid hormone
• Trophoblastic disease
• Conditions that elevate total T₄ (but not FT₄ as significantly) due to increased proteins (TBG, prealbumin, and albumin) such as pregnancy, genetically increased TBG, acute intermittent porphyria, and primary biliary cirrhosis
• Euthyroid patients with familial dysalbuminemic hyperthyroxinemia
• Drugs (eg, amiodarone, estrogens, clofibrate, methadone)

Limitations

---

• Although FT₄ methods are designed to be unaffected by changes in binding protein concentrations, extremes of TBG, prealbumin, and albumin can interfere with immunoassay results (Dufour, 2007). This includes changes that occur during normal pregnancy (increased estrogen causes higher levels of TBG which in turn causes increased T₄ and T₃ levels) (Lee, 2009; Spencer, 2013).
• In highly sensitive single- or double-antibody immunoassays, the presence of circulating endogenous antibodies including autoantibodies, heterophile antibodies, and rheumatoid factors can cause misleading results (Després, 1998).
• This assay will not detect T₃ thyrotoxicosis.
• Discrepancies in free T₄ levels between methods are recognized.
• Most FT₄ immunoassays yield falsely high values with familial dysalbuminemic and transthyretin-associated hyperthyroxinemia due to altered protein structures with increased affinity for thyroxine and/or triiodothyronine. FT₄ is normal when measured by direct techniques (eg, equilibrium dialysis) (Spencer, 2013).
• Both fractionated and unfractionated heparin may cause artificial elevations of FT₄ due to T₄ displacement from albumin by generated free fatty acids. This may reflect an in vitro reaction or continuation of in vivo to in vitro. It might be advantageous to order thyroid function testing prior to heparin administration or consider measurement of total T₄.
• High circulating levels of biotin can interfere with thyroid function assays, most commonly causing falsely high levels of T3, free T3 and free T4 and falsely low levels of TSH, leading to an incorrect diagnosis of hyperthyroidism or conclusion that thyroid hormone dose is too high. Results inconsistent with clinical picture should be investigated (Li 2017).
• Total T₄ alone is not recommended as an initial screening test (Dayan, 2001).
• This assay will not detect T₃ thyrotoxicosis.
• As 99% of T₄ is bound to plasma proteins (TBG, prealbumin, albumin), changes in the concentrations of these proteins (via disease, pregnancy, malnutrition) will falsely increase or decrease results. FT₄ (active fraction) is much less sensitive to changes in the binding proteins.

References

---

Diagnostic Role

---

Combining the sTSH test with the FT₄ will accurately determine thyroid gland functioning in most clinical situations. An elevated TSH with low FT₄ indicates primary hypothyroidism due to primary thyroid gland dysfunction. A low TSH and low FT₄ indicate central hypothyroidism due to a problem involving the pituitary, or less commonly, the hypothalamus. A low TSH with an elevated FT₄ indicates primary hyperthyroidism.

Alias

---

• free thyroxine (free t4)
• t4, free (thyroxine)
• thyroxine, free (t4)

Test Setup Days

---

Monday through Friday PM shift

CPT

---

84439 Limited Coverage Test For Medicare.
Advance Beneficiary Notice Of Non-Coverage (ABN) Required
If Diagnosis Is Not Covered.
Frequency Limit Test For Medicare.
Advance Beneficiary Notice Of Non-Coverage (ABN) Always
Required For Frequency.
If Free Thyroxine Is Measured, It Is Not Considered
Appropriate To Measure T4 Total Or T3 Uptake.
Medicare Payment For T4 Total And T3 Uptake Will Be Denied
If Ordered On The Same Day As Measured Free Thyroxine.
LOINC: 3024-7

Reference Range

---

AGE 0-3 DAYS: 0.70-2.70 NG/DL
4 DAYS-1 MONTH: 0.80-3.10 NG/DL
2-23 MONTHS: 0.50-2.30 NG/DL
2-6 YEARS: 0.90-1.80 NG/DL
7-11 YEARS: 0.90-1.70 NG/DL
12-19 YEARS: 0.90-1.60 NG/DL
>=20 YEARS: 0.80-1.90 NG/DL
NOTE:EXPECTED REFERENCE RANGE FOR FREE T4 VS. TSH
FOR PATIENTS >=20 YEARS IS AS FOLLOWS.
FREE T4 – 0.80-1.90 NG/DL . . TSH – 0.400-1.000 MIU/ML
FREE T4 – 0.80-1.70 NG/DL . . TSH – 1.000-4.100 MIU/ML

UNIT CODE	UNIT CODE NAME	ANALYTE	GENDER	AGE	REFERENCE RANGE	Units of Measure
2823	FREE T4	FT4	NOT SPECIFIED	0Y	0.80-1.90	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	3D	0.70-2.70	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	1M	0.80-3.10	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	23M	0.50-2.30	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	6Y	0.90-1.80	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	11Y	0.90-1.70	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	19Y	0.90-1.60	NG/DL
2823	FREE T4	FT4	NOT SPECIFIED	150Y	0.80-1.90	NG/DL
2823	FREE T4	FT4	MALE	0Y	0.80-1.90	NG/DL
2823	FREE T4	FT4	MALE	3D	0.70-2.70	NG/DL
2823	FREE T4	FT4	MALE	1M	0.80-3.10	NG/DL
2823	FREE T4	FT4	MALE	23M	0.50-2.30	NG/DL
2823	FREE T4	FT4	MALE	6Y	0.90-1.80	NG/DL
2823	FREE T4	FT4	MALE	11Y	0.90-1.70	NG/DL
2823	FREE T4	FT4	MALE	19Y	0.90-1.60	NG/DL
2823	FREE T4	FT4	MALE	150Y	0.80-1.90	NG/DL
2823	FREE T4	FT4	FEMALE	0Y	0.80-1.90	NG/DL
2823	FREE T4	FT4	FEMALE	3D	0.70-2.70	NG/DL
2823	FREE T4	FT4	FEMALE	1M	0.80-3.10	NG/DL
2823	FREE T4	FT4	FEMALE	23M	0.50-2.30	NG/DL
2823	FREE T4	FT4	FEMALE	6Y	0.90-1.80	NG/DL
2823	FREE T4	FT4	FEMALE	11Y	0.90-1.70	NG/DL
2823	FREE T4	FT4	FEMALE	19Y	0.90-1.60	NG/DL
2823	FREE T4	FT4	FEMALE	150Y	0.80-1.90	NG/DL