Is it appropriate for subordinates to express their hard work to their leaders?

Inappropriate.

Thank you for your hard work. This word is usually a condolence word spoken by a superior to a subordinate. If a subordinate says this to his superior, it will feel like a joke. If the relationship between two people is very good, this word cannot be used in other situations. If you say it in front of others, if you have a normal relationship, you must never say it out loud. This time it will make your superiors feel strange, or even leave a bad impression.

If you have to say something, you can say I'm sorry to bother you, or I'm sorry for bothering you. In short, you need to show your concern for the leader and also show that you are a subordinate.

Extended information

Things to note when communicating with leaders:

1. Listen. When your boss is speaking, eliminate all thoughts that make you nervous and listen attentively. Keep your eyes on him, don't bury your head blankly, and make some notes if necessary. After he finishes speaking, you can think about it for a moment or ask a question or two to really understand what he meant.

2. Simplicity. Time is life and a valuable asset for managers. Being concise and quick in doing things is the basic quality of a staff member. Simplicity means reporting to your boss selectively, straightforwardly, and very clearly.

3. Talk about tactics. If you want to propose a plan, you must carefully organize your arguments and reasons, and try your best to present its advantages so that your boss can easily accept it. It would be even better if you can propose multiple options for him to choose from. You can list the pros and cons of various options for him to weigh.