Free T4: What It Means for Your Health
Free T4 (free thyroxine) measures the unbound, biologically active portion of thyroxine in your blood. While most T4 circulates bound to proteins, only the free fraction (about 0.03%) is available to enter cells and exert metabolic effects. Free T4 is a more accurate measure of thyroid function than total T4 because it is not affected by changes in binding protein levels (which occur in pregnancy, estrogen use, and liver disease).
Reference Range
Reference ranges may vary between laboratories, age groups, and sexes. The values shown are general adult reference ranges. Always compare your results to the specific range provided by your testing laboratory.
What Is Free T4?
Free T4 (free thyroxine) measures the unbound, biologically active portion of thyroxine in your blood. While most T4 circulates bound to proteins, only the free fraction (about 0.03%) is available to enter cells and exert metabolic effects. Free T4 is a more accurate measure of thyroid function than total T4 because it is not affected by changes in binding protein levels (which occur in pregnancy, estrogen use, and liver disease).
Commonly included in:
What Does High Free T4 Mean?
Elevated Free T4 confirms hyperthyroidism when TSH is low. Causes include Graves' disease, toxic multinodular goiter, thyroiditis (temporary hormone release from an inflamed gland), or excessive thyroid medication. Symptoms include weight loss, anxiety, rapid or irregular heartbeat, and heat intolerance.
What Does Low Free T4 Mean?
Low Free T4 with elevated TSH confirms primary hypothyroidism. With normal or low TSH and low Free T4, central hypothyroidism (pituitary or hypothalamic dysfunction) should be considered. Symptoms include fatigue, weight gain, cold intolerance, constipation, and cognitive slowing.
Related Biomarkers in Thyroid Function
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This information is for educational purposes only and is not intended as medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult a qualified healthcare professional for interpretation of your blood test results and before making any health decisions.