We recently got TSH hormone from Sigma. On the first day, we ran our assay, and the results were fine. On the second day, the activity has somehow reduced by 90%. How is this possible?

We reconstituted the lyophilised hormone in PBS and diluted it to several aliquots and stored them in the freezer. We used one stock and prepared 1 unit concentration sample, which gave the corresponding correct reading (1 unit) on day 1. We then stored the used stock at 4 deg overnight. Next day, we ran 4 and 8 unit samples, and reading was around 0.5-0.7 units.

We are using ELISA kit by Calbiotech company.