Thyroid hormones assays (T4 and T3) are limited to the free forms with hormonal activity, the only ones useful for the assessment of thyroid function. Free thyroxine assays are part of prescribed parameters by the physician once known plasma TSH concentration. A reference method exists, but immunoassays are the most commonly technics used in current practice. After a reminder of the iodothyronine biochemistry and physiology, the authors discuss preanalytical and analytical steps in detail, focusing on the possible pitfalls.
Keywords: immunoassay; thyreostimulin; thyroid hormones.