Thyroxine (T4) is a hormone produced by the thyroid gland. It is sometimes called total thyroxine because it includes both free T4 and T4 bound to proteins. Thyroxine is only one-tenth as potent as the other major thyroid hormone, triiodothyronine (T3). Moreover, the portion of thyroxine that is biologically active is free T4, that is, the portion of T4 that is not bound to proteins in the blood. Greater than 99% of thyroxine is bound to serum proteins. Thyroxine acts on almost every cell in the body. It sets the metabolic tone of cells. Thyroxine is critically important for the growth and development of fetuses, neonates, and children. The thyroid gland produces and stores thyroxine until it is needed for release. The thyroid gland releases T4 when it is stimulated by thyroid-stimulating hormone, also known as TSH or thyrotropin. Free thyroxine is often more useful than total thyroxine (total T4) in assessing thyroid function, but total T4 can be a useful biomarker in some cases.

Normal Ranges for Total T4:

Adults: 4.5-111.7mcg/dL

0-5 days: 5.0-18.5 mcg/dL

6 days-2 months: 5.4-17.0 mcg/dL

3-11 months: 5.7-16.0 mcg/dL

1-5 years: 6.0-14.7 mcg/dL

6-10 years: 6.0-13.8 mcg/dL

11-19 years: 5.9-13.2 mcg/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

https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/8724

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