Generally it would not be necessary to have very specific details about your procedure to approve the hardship distribution for medical purposes. However, this can be a gray area.

Your plan sponsor must insure that they are only allowing proper hardship distributions from the plan. The prudent approach for them is to keep paperwork on file that shows they used a proper method to determine that in the event of an audit. Where this can get gray is that there is not the clearest of lines between what is absolutely needed and what is not.

For example, if a person was in a car accident and had medical bills, the plan sponsor may determine that they accept that a car accident was the cause and they see a medical bill for the amount which provides the amount. They may not require a police report of the accident. It's really up to them.

For a medical procedure, the plan sponsor may decide they need to see a detailed bill or they may decide a roll-up view is acceptable. Again, it's up to them. Privacy rules generally don't apply to a 401(k) plan, although your doctor is unlikely to release details about you so, for your employer to get any details, it would most likely have to come from you.

The only way you will know for sure is to proceed with the process, get the paper work that you need to provide, and see what they require.

If you received a hardship loan for medical bills; paid the taxes on the loan. Is this something you may claim back on your incom tax returns?