Many people perspire heavily (body chemistry) or when stressed. STAY AWAY FROM SYNTHETIC SHIRTS! BUY ONLY 100% COTTON OR AT LEAST A COTTON/SYNTHETIC BLEND. Synthetics don’t let the body breath.
You need to get to the problem to stop the staining. This is an American non-prescription under-arm roll-on called “Certain dri” to be used every other day and it works according to the chat boards.
Make a paste out of Oxy-clean and smear a light paste on all areas in the armpit area of the shirt. Leave on for an hour or so and then wash as usual.
We have a product in Canada called “Gal Friday” (don’t know if you can get in the States) it’s liquid and gets many stains out. It’s sure saved on my dry-cleaning bill! There is also a product out called “Yellow Out” that DOES get stains out.

 

Persperation reacts with minerals in the water, causing yellow or brownish stains on clothes. This is especially true of whites. Bleaching only makes the problems worse. To get these rust stains out I use a product full strength called Rust Stain Remover from Whink. You can see the stains (rust) dissolve before your eyes. Do not get this on your washing machine or sink, however, since it takes off the glaze. It is also expensive and does not work as well when diluted with water.
For general rust caused by hard water I use “oxalic acid”. The best and cheapest place to find this is in boating stores (call first). It is used to clean up teak wood on boats. Oxalic acid comes in powered form, but be aware that it is a poison so store it carefully. Every once in awhile I put a couple tablespoons in the empty washing machine and run it through a cycle. This cleans up all the “rust” that has built up in your washing machine. You will see the sides of the washer go from yellow to white. Then run a load of white “rusty” clothes through with another couple tablespoons. Let them soak a little.
The oxalic acid also cleans up the inside of a yicky looking dishwasher and makes it look like new, but run it through an EMPTY dishwasher, since it is poison. Put some in the water storage comparment of a toilet and it will clean out all the mold and slime (but make sure your dogs do not drink from the toilet!!)