In my personal and clinical experience with working with over 2,000 people with Hashimoto's, I can tell you that diet is the foundation for any successful Hashimoto's treatment.
And here's why: Hashimoto's is an autoimmune disease and an estimated 70-80% of the immune system is found in the gut. Dietary proteins have a huge impact on your immune system and can trigger lots of different reactions.
In addition, leaky gut or intestinal permeability is one of the factors that leads to the initiation and progression of Hashimoto's in the first place.
Here's some great information on the best diet for Hashimoto's, various foods that may cause problems, and some major issues that can prevent you from getting better such as losing oral tolerance, SIBO (Small Intestine Bacterial Overgrowth), etc.