Free T4 (Thyroxine)
Definition
Free T4 is the unbound, bioavailable form of thyroxine, the primary hormone produced by the thyroid gland. T4 is a prohormone that must be converted to the active form, T3, in peripheral tissues. Free T4 levels reflect thyroid gland output and conversion capacity.
Why It Matters
Free T4 levels help assess thyroid hormone production. Low free T4 with elevated TSH confirms hypothyroidism, while low free T4 with normal TSH may indicate pituitary dysfunction. Monitoring free T4 alongside free T3 reveals whether the body is effectively converting T4 to active T3.
Related Conditions
Related Modalities
Frequently Asked Questions
Total T4 measures both bound and unbound hormone, while free T4 measures only the biologically active, unbound form. Free T4 is a more accurate reflection of thyroid hormone availability to tissues and is preferred in clinical assessments.
Yes. Normal free T4 doesn't guarantee adequate free T3 levels. Poor T4-to-T3 conversion, high reverse T3, or cellular thyroid resistance can cause symptoms despite normal T4. Comprehensive testing including free T3 and reverse T3 provides better insight.
Dr. Sanika Kshirsagar, ND
Doctorate of Naturopathic Medicine (ND)
Bastyr University, Kenmore, WA
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