How long have you had the shirt? Since all shirts of all colours will have stiff patches of cloth around the armpit which seem to have been starched, when in actual fact over the time the deodorant will have reacted with the washing powder to give a starch effect. In which case it's time to change the shirt; if the shirt is relatively new and can withstand dry cleaning, then I suggest taking it to a professional dry cleaners to get it cleaned.
DISCLAIMER: The messages on this forum and any links to external websites posted within the forum express the views of the respective authors. They do not necessarily represent the views of the administrators or moderators. Any advice posted here is for support purposes only. It is not intended as a substitute for advice by a qualified health professional.