Hypo is just a technical term for T3 levels under a certain point but human body function is not absolute, its not black and white, its not working and broken. There are degrees between optimal and suboptimal.
You may never have been clinically "hypo" BUT the fact that you added Iodine and are seeing body composition changes then thats a direct indication you were not functioning optimally.
There are countless studies linking hormone levels to thyroid function. With that being said if your thyroid was not functioning optimally your may have had lower than optimal testosterone levels as well. Improving thyrodi function by taking iodine/selenium means your body is more efficient at losing fat and gaining muscle as well as optimizing your sex hormone levels.