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Free T4 (FT4, Free Thyroxine) is one of the main hormones secreted by the thyroid gland. A small portion circulates in its free, unbound form, which enters cells directly to regulate metabolism, energy use, and body temperature. Unlike total T4, FT4 more accurately reflects thyroid function.
A venous blood sample is collected to measure serum FT4 levels. Doctors interpret the results together with FT3, TSH, and clinical symptoms.
Q1: What is the difference between FT4 and FT3?
A: FT4 is the main thyroid hormone at higher concentration, while FT3 is more active and largely converted from FT4.
Q2: What does high or low FT4 mean?
A: High FT4 usually indicates hyperthyroidism, while low FT4 suggests hypothyroidism or reduced thyroid function.
Q3: Why combine FT4 with TSH testing?
A: FT4 reflects thyroid hormone secretion, while TSH shows pituitary regulation. Together they provide a clearer diagnosis.