Free Thyroxine

Metabolic Health

A healthy result should fall into the range 0.6 - 1.2 ng/dL, or 0.60 - 1.20 pmol/L.

Thyroxine (sometime called T4) is a hormone produced by the thyroid gland. The term “free thyroxine” means measured thyroxine that is not bound to proteins in the blood. Thyroxine is only one-tenth as potent as triiodothyronine (T3). That being said, thyroxine acts on almost every cell in the body, helping to setting the metabolic tone of cells. Thyroxine is important for growth and development, especially in fetuses, neonates, and children. The thyroid gland produces and stores thyroxine until it is needed for release. Thyroid-stimulating hormone, also known as TSH or thyrotropin, is produced in the pituitary gland. TSH stimulates the thyroid gland to produce and release of T4. Greater than 99% of thyroxine is bound to serum proteins, but the portion that is not, free thyroxine, is the biologically active portion. In most cases, free thyroxine (also known as free T4) is good indicator of thyroid function, along with TSH and other measures. Free thyroxine is often more useful than total thyroxine in assessing thyroid function.

Normal Ranges for Free T4:

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

 

Sources:

http://www.ncbi.nlm.nih.gov/books/NBK285568/

http://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function

http://www.ncbi.nlm.nih.gov/pubmed/12625976

http://www.questdiagnostics.com/testcenter/TestDetail.action?ntc=866

http://dx.doi.org/10.1001/jama.1990.03440110095035

http://www.uptodate.com/contents/thyroid-function-in-nonthyroidal-illness

http://www.ncbi.nlm.nih.gov/pubmed?term=7920008

http://www.ncbi.nlm.nih.gov/pubmed/6687730

http://www.uptodate.com/contents/overview-of-thyroid-disease-in-pregnancy

http://www.uptodate.com/contents/diagnosis-of-and-screening-for-hypothyroidism-in-nonpregnant-adults

https://www.uptodate.com/contents/diagnosis-of-hyperthyroidism

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