Thyroxine, or T4, is a hormone produced by the thyroid gland in the neck. It regulates numerous bodily functions, including how the body uses energy. Understanding T4’s purpose and normal levels is important for assessing overall health and metabolic balance.
The Role of Thyroxine (T4) in Your Body
Thyroxine (T4) is one of two primary hormones released by the thyroid gland, alongside T3 (triiodothyronine). T4 makes up about 80% of thyroid hormone output. Though the main circulating thyroid hormone, T4 is a “prohormone” that converts to its active form, T3, primarily in organs like the liver and kidneys, to exert most effects.
T4 and its converted T3 regulate metabolism, controlling how the body uses energy. These hormones influence heart rate, body temperature, and digestive tract function. Beyond metabolism, T4 contributes to brain development, bone maintenance, muscle control, and cellular function. Proper T4 levels maintain energy, cognitive function, and healthy bodily systems.
Deciphering Your T4 Test Results
T4 levels are measured with a blood test to assess thyroid function. Two main types are Total T4 and Free T4 (fT4). Total T4 measures both protein-bound and free T4 in the blood. Free T4 is considered more accurate because it measures the unbound, active hormone available to tissues, which is less than 1% of total T4.
Example normal ranges for adults vary between laboratories. A Total T4 level typically falls between 5.0 and 12.0 micrograms per deciliter (mcg/dL). For Free T4, a typical range is 0.8 to 1.8 nanograms per deciliter (ng/dL). Actual normal values differ by laboratory and individual health context, so a healthcare provider is essential for interpreting results.
T4 levels outside the normal range can suggest an underlying thyroid condition. Higher-than-normal T4 or Free T4 levels may indicate hyperthyroidism (an overactive thyroid). Conversely, lower-than-normal T4 levels may suggest hypothyroidism (an underactive thyroid). T4 levels are often evaluated alongside Thyroid Stimulating Hormone (TSH) levels for a comprehensive assessment, as TSH regulates T4 production.
Common Influences on T4 Levels
Several factors can influence T4 levels, sometimes causing them to fall outside typical ranges without a primary thyroid disorder. Medications significantly influence T4 levels. For instance, estrogen-containing drugs (e.g., oral contraceptives, hormone replacement therapy) can increase Total T4 by affecting binding proteins. Certain anticonvulsants (e.g., phenytoin, carbamazepine, phenobarbital) can accelerate thyroid hormone metabolism, potentially lowering T4. Amiodarone, a heart medication, and biotin supplements can also interfere with T4 measurements or metabolism.
Physiological states also cause T4 fluctuations. Pregnancy, for example, naturally elevates Total T4 due to increased thyroid hormone binding proteins, even with normal thyroid function. Acute illnesses can temporarily lower T4 levels, a phenomenon called non-thyroidal illness syndrome. T4 levels can also be affected by age, with different reference ranges for children and adolescents. While stress or nutritional deficiencies can have an impact, medications and major physiological changes are more common influences.