When I searched the web last year I couldn't find anything about how to store Estriol in Summer temps.
I've since found this on the web. Do not store in a fridge, it should be stored at room temp between 15c and 30c. Do not store over 40c.
I live in a new eco home that can be like a furnace in summer, as an experiment I left my half tube of Estriol at room temp of over 28c and it had no affect on the cream whatsoever. I placed my new tube of Estriol in the fridge and when I took it out I knew this was wrong as the tube felt too cold.
I am now happy to leave my tube at room temp and not worry anymore.