How to hide a custom user field in user profile?
I think Field Permissions is what you are looking for. Here is a quote about it (from the module's project page):

... allows site administrators to set field-level permissions to edit, view and create fields on any entity.
Features:

Enable field permissions on any entity, not just nodes.
Role-based field permissions allowing different viewing patterned based on what access the user has.
Author-level permissions allow viewing and editing of fields based on who the entity owner is.
Permissions for each field are not enabled by default. Instead, administrators can enable these permissions explicitly for the fields where this feature is needed.
Field permissions overview