was ist ft3 und ft4 — which blood markers matter?

FT3 (free triiodothyronine) and FT4 (free thyroxine) are the two active thyroid hormones measured in blood. FT4 is produced by the thyroid gland and converted to the more potent FT3 in peripheral tissues (mainly liver and kidney). Measuring the free (unbound) fraction is clinically important because it reflects the hormone actually available to cells. Which blood markers matter: TSH (thyroid-stimulating hormone) is the most sensitive screening parameter. For suspected thyroid disorders, FT4, FT3, and possibly antibodies (TPO-Ab, Tg-Ab, TRAb) are added. Isolated FT3/FT4 without TSH is not meaningful because the hypothalamic-pituitary-thyroid axis regulates feedback. Caveat: reference ranges vary by lab and method; interference from medications (e.g., biotin, glucocorticoids) or non-thyroidal illness (euthyroid sick syndrome) can occur. The evidence for clinical utility of these markers is human-strong (endocrine guidelines). A DNA test cannot predict thyroid hormone levels – they must be measured directly in blood.