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
Decreased free thyroxine levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Abnormally low thyroxine may cause symptoms of hypothyroidism including weakness and fatigue, cold intolerance, shortness of breath, weight gain, constipation, cognitive problems, dry skin, hoarseness, and swelling (edema).
Some specific causes of low free thyroxine are:
- Hypothyroidism
- Chronic thyroiditis
- Subacute thyroiditis
- Nephrosis
- Congenital thyroid agenesis, dysgenesis, or defects in hormone synthesis
- Synthetic T3 treatment
- Medications anabolic steroids such as glucocorticoids, phenytoin, carbamazepine, phenobarbital
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Elevated free thyroxine levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or thyroxine toxicosis. Elevated free thyroxine may cause symptoms of hyperthyroidism including excessive appetite, anxiety, heart palpitations, sweating, shortness of breath, weight loss, and intolerance to heat.
Some specific causes of high free thyroxine are:
- Hyperthyroidism
- Euthyroid hyperthyroxinemia
- TSH-mediated hyperthyroidism
- Acute thyroiditis
- Pregnancy
- Familial dysalbuminemic hyperthyroxinemia
- Medications (e.g. estrogens, tamoxifen, opioids)
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