Biomarkers · Chemical
Thyroxine is also called T4. T4 is a hormone that your thyroid gland makes.
Check orderable catalog codes, specimen types, fasting requirements, and demographic reference ranges across laboratories.
Laboratory assay overview
This profile is used for basic diagnostic laboratory testing for evaluation of post-COVID conditions.
Labcorp test directory, test code 165065
Laboratory assay overview
The T4 Free (FT4) test measures serum thyroxine (T4) that is not bound to thyroid hormone-binding proteins (thyroid hormone-binding globulin [TBG], transthyretin, albumin) and is thus readily available to target cells. This test is used with thyroid-stimulating hormone measurement (TSH, test code 899) in the diagnosis of hyperthyroidism (usually displaying elevated FT4) and hypothyroidism (low FT4), including congenital and acquired hypothyroidism. The FT4 test is used to assess thyroid function, monitor thyroid conditions, and assess treatment effectiveness. It has largely replaced the total T4 test (test code 867), which measures both bound and free T4, because the FT4 test is less affected by levels of thyroid hormone-binding proteins. This test uses a T4 analog, rather than equilibrium dialysis, to determine the amount of free T4. It therefore differs from the T4 Free, Direct Dialysis test (test code 35167). In the general population, the analog FT4 test is most useful as a follow-up test when TSH results are abnormal. It may also be used instead of TSH testing to assess thyroid function in patients with (1) known pituitary abnormalities; (2) chronically suppressed TSH, as often seen in patients for months after successful treatment for Graves disease [1]; or (3) a need to assess thyroid hormone dose changes sooner than the 6-8 weeks required for TSH to reach a new stable level. When the levels of thyroid hormone-binding proteins are very abnormal (eg, congenital TBG deficiency or excess; preterm infants [2]) or the affinity of binding proteins is altered [3] (eg, familial dysalbuminemic hyperthyroxinemia), the T4 Free, Direct Dialysis test will provide more accurate results. References 1. Ross DS. N Engl J Med 2011;364:542-550. 2. Deming DD et al. J Pediatr. 2007;151:404-408. 3. Refetoff S. Abnormal Thyroid Hormone Transport. [Updated 2015 Jul 15]. In: Feingold KR, Anawalt B, Boyce A, et al, editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000.
Quest Diagnostics population reference ranges
Quest Diagnostics calculates these statistical intervals for this specific assay and demographic group, not as universal optimal targets.
| Population | Range |
|---|---|
| <1 Month | Not established |
| 1-23 Months | 0.9-1.4 ng/dL |
| 2-12 Years | 0.9-1.4 ng/dL |
| 13-20 Years | 0.8-1.4 ng/dL |
| >20 Years | 0.8-1.8 ng/dL |
<1 Month Not established 1-23 Months 0.9-1.4 ng/dL 2-12 Years 0.9-1.4 ng/dL 13-20 Years 0.8-1.4 ng/dL >20 Years 0.8-1.8 ng/dL
Quest Diagnostics test directory, test code 866
Labcorp test directory, test code 224576
Laboratory assay overview
Free T4 may be indicated when binding globulin (TBG) problems are perceived, or when conventional test results seem inconsistent with clinical observations. It is normal in subjects with high thyroxine-binding globulin hormone binding who are euthyroid (ie, free thyroxine should be normal in nonthyroidal diseases). It should be normal in familial dysalbuminemic hyperthyroxinemia.
Labcorp population reference ranges
Labcorp calculates these statistical intervals for this specific assay and demographic group, not as universal optimal targets.
| Population | Range |
|---|---|
| Age | Range (ng/dL) |
| 0 to 3 d | 0.66−2.71 |
| 4 to 30 d | 0.83−3.09 |
| 31 d to 12 m | 0.48−2.34 |
| 13 m to 5 y | 0.85−1.75 |
| 6 to 10 y | 0.90−1.67 |
| 11 to 19 y | 0.93−1.60 |
| >19 y | 0.82−1.77 |
See table.1 Age | Range (ng/dL) 0 to 3 d | 0.66−2.71 4 to 30 d | 0.83−3.09 31 d to 12 m | 0.48−2.34 13 m to 5 y | 0.85−1.75 6 to 10 y | 0.90−1.67 11 to 19 y | 0.93−1.60 >19 y | 0.82−1.77
Labcorp test directory, test code 001974
We compile every detail above directly from named laboratory directories and published clinical studies. Laboratories calculate standard reference intervals from the statistical distribution of tested populations for a specific assay method and demographic group, rather than setting universal optimal targets.
Always review your personal lab results with a licensed clinician, or track your lab history and health goals in Aumnivida.
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