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
Decreased free T4 levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Abnormally low T4 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 T4 are:
- Chronic thyroiditis
- Subacute thyroiditis
- Congenital thyroid agenesis, dysgenesis, or defects in hormone synthesis
- Synthetic T3 treatment
- Medications anabolic steroids such as glucocorticoids, phenytoin, carbamazepine, phenobarbital
Elevated free T4 levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or T4 toxicosis. Elevated free T4 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 T4 are:
- Euthyroid hyperthyroxinemia
- TSH-mediated hyperthyroidism
- Acute thyroiditis
- Familial dysalbuminemic hyperthyroxinemia
- Medications (e.g. estrogens, tamoxifen, opioids)
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