A healthy result should fall into the range 0.7 - 1.53 ng/dL, or 9.01 - 19.69 pmol/L.

Thyroxine or T4 is a hormone produced by the thyroid gland. The term “free T4” means measured T4 that is not bound to proteins in the blood. T4 is about one-tenth as potent as T3 in humans. Nonetheless, T4 acts on almost every cell in the body, helping to setting the metabolic rate of the cell. T4 is important for growth and development, especially in fetuses and children. The thyroid gland produces and stores T4 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 T4 is bound to a serum proteins, but the portion that is not, free T4, is the biologically active portion. In most cases, free T4 is good indicator of thyroid function, along with TSH and other measures. Free T4 is often more useful than total T4 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-hyperthyroidis

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