I need to storage my plasma samples at -80 °C until I'll collect all of them. I'll measure T3, T4, cortisol and so on by ELISA, but i need to know the stability of those analites from plasma at -80°C
We stered samples for over 5 years and did not observe any problem measuring the compounds you mention. However, this might be different for protein hormones.
indeed as F. de Jong said there is no problem for freezing your samples at -80 C for several years. We did it too and did not find any differences. Important it must be plasma, avoid temperature changes and storage-tubes should be sealed very well.
We have experience with samples that were over 30 years old, without a problem. Up to 5 freeze-thaw cycles for testosterone, estradiol, progesterone, cortisol and corticosterone.