I am glad to hear that at age 21 you have the wisdom to fast and take your health in your own hands. Taking medication while fasting with water only is a bad idea. Too much water is also a load on your body.
Drink water as per thirst. Your body will heal everything it needs to. Sleep will be erratic during a fast, not to worry.
After you complete your fast the most simple way to help Hypothyroid is to take Iodine. This is what helped me in just one day, my thyroid started acting normal, this was many years ago. However, you have to follow the protocol.
Please read the messages on Iodine supplementation forum and learn about how to take Iodine.
Thyroid is connected to many diseases, especially Depression and anxiety.