Stability of thyroxine and triiodothyronine in biological fluids

The stability of thyroxine and triiodothyronine in serum has been investigated. Apparent levels of total thryroxine, as determined by two different protein-binding assays employing thyroxine-binding globulin as the binding protein, increased significantly in serum and plasma samples stored at room temperature and were signficantly lower in haemolysed samples. Values did not change significantly in samples stored at 4 degrees C, nor in samples stored at room temperature when determined by radioimmunoassay. Total triiodothyronine levels, as determined by radioimmunoassay, fell slightly on storage. Failure to appreciate the effect of storing samples at room temperature on apparent levels of total thyroxine, as determined by some protein-binding assays, could lead to an incorrect assessment of thyroid status.