Free T4
Measures unbound thyroid hormone T4 to assess metabolism and energy production.
What This Test Measures
Specimen & Collection
Preparation
No fasting is required for this test. Inform your healthcare provider of any thyroid medications or biotin supplements, as these may affect results.
Biomarkers Included (3)
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Free T4, or thyroxine, is the active thyroid hormone that regulates metabolism, energy, and body temperature. This test measures the unbound form of T4 circulating in your blood—the portion your body can actually use. Low free T4 suggests an underactive thyroid (hypothyroidism), causing fatigue and weight gain. High free T4 indicates an overactive thyroid (hyperthyroidism), causing rapid heartbeat and anxiety. Free T4 is often ordered alongside TSH to assess thyroid function comprehensively.
The free T4 index is a calculated measure of thyroid hormone activity that estimates the amount of thyroxine (T4) available to affect your metabolism. Unlike total T4, it accounts for proteins that bind thyroid hormones, providing a more accurate picture of active thyroid hormone. This test helps evaluate thyroid function and diagnose hyperthyroidism or hypothyroidism when TSH alone may be unclear.
This panel measures both thyroid stimulating hormone (TSH) from the pituitary gland and free T4 (thyroxine), the active thyroid hormone. Together, these tests provide a comprehensive assessment of thyroid function. TSH shows whether the pituitary is signaling the thyroid correctly, while free T4 measures the actual active hormone. This combination is more reliable than either test alone for diagnosing thyroid disorders.
Included in Panels
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This information is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult with a qualified healthcare provider regarding any medical condition or treatment plan.
Lab results should be interpreted by a licensed healthcare provider in the context of your complete medical history. Processing times may vary by laboratory. Charges will not be submitted to insurance, Medicare, or Medicaid. Direct-access lab testing is not available in NY, NJ, or RI.