If a recipe calls for ground thyme, can I substitute it with italian seasoning?

I am making italian roast beef sandwiches for dinner tonight and it calls for me to simmer the beef for 3-4 hours in a dutch oven. The recipe calls for ground thyme but all I have is italian seasoning flakes, which has thyme and oregano in it. Would this substitution work, and would I need to change anything in the recipe, which calls for me to add 1 tsp. of thyme when I begin cooking the beef? Thanks.