While I agree with the general rule, that nobody should know your credentials, in practical terms, some times it may be required, if you want the help, and you don't want to sit around re-typing your password for the tech.
Perhaps I am miss-reading your question, but it appears you are talking about an external service? Most of the other answers seem to be missing the point, and assuming the IT admin actually has administrative access.
Within my network many external services are in use, and I have no admin access on, and I do not even have an account at all. If one of my users needs me to assist them in resolving an issue, I have no ability to reset passwords or force a change after the fact. The only way for me to resolve the problem is with their credentials. For smaller problems having the users simply type in their credentials is fine, but for really complex ones that have to do with the interactions of many systems, this just is not practical.
Ideally, the person would change their password before and/or after I have used their credentials to resolve problems with an external service, but users typically really dislike this. If you are concerned simply mention it to the IT admin. I am sure he wouldn't mind you changing the password before he works on it.
In the case of small companies, some passwords for external services belong to the organization and not the individual. For example, lets say your web site was hosted on some bargin-basement cheap hosting provider that only provides a single account to manage the content. In a perfect world you would pay for better service that provides individual accounts to each user, but in it reality it is very common to have a set of shared credentials for external services. If you want your tech to help you figure out why some setting is screwed up, or why some file is corrupt, then he will need access, and if you only have the option for a single account, you may simply have no choice but to share the password.