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Free Thyroid Hormone

Meaning

Free thyroid hormone refers to the small, unbound fraction of the major thyroid hormones, thyroxine (T4) and triiodothyronine (T3), circulating in the bloodstream that is not bound to plasma proteins like Thyroxine-Binding Globulin (TBG) and albumin. This unbound concentration is the biologically active component, as only the free hormone can passively diffuse into target cells to regulate gene expression and metabolic rate throughout the body. Clinically, measuring the free T4 (FT4) and free T3 (FT3) levels is considered the most reliable method for accurately assessing thyroid gland function and diagnosing states of hyperthyroidism or hypothyroidism.