Nails can signal all sorts of issues with the body, like autoimmune disease, diabetes, thyroid disease, etc. The most common direct cause is fungal infection. The appearance usually changes, and you didn't describe a color change.
A fungal infection may start as spots of brownish discoloration or the nail may thicken and get bumpy. It can get a little tender.
Use 100% tea tree oil on all the toenails morning and night. The bad nail has to grow out. Clean your tools for trimming the nails. Avoid keeping your feet in moisture. Wash socks and underwear in hot water and add baking soda to kill fungus. It spreads easily.
Get checked out for diabetes, thyroid problems, and circulatory problems. Check for food allergies.
This site complies with the HONcode standard for trustworthy health information.
The Content on this Site is presented in a summary fashion, and is intended to be used for educational and entertainment purposes only. It is not intended to be and should not be interpreted as medical advice or a diagnosis of any health or fitness problem, condition or disease; or a recommendation for a specific test, doctor, care provider, procedure, treatment plan, product, or course of action. Med Help International, Inc. is not a medical or healthcare provider and your use of this Site does not create a doctor / patient relationship. We disclaim all responsibility for the professional qualifications and licensing of, and services provided by, any physician or other health providers posting on or otherwise referred to on this Site and/or any Third Party Site. Never disregard the medical advice of your physician or health professional, or delay in seeking such advice, because of something you read on this Site. We offer this Site AS IS and without any warranties. By using this Site you agree to the following Terms and Conditions. If you think you may have a medical emergency, call your physician or 911 immediately.