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:
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
Decreased 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 thyroxine are:
Elevated thyroxine levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or thyroxine toxicosis. Elevated 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 thyroxine are:
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