My tsh is 18 and antibodies are more than 600, what are these antibodies, and why did it increase?

Hypothyroid: You likely have hashimoto's thyroiditis as the cause of your low thyroid. I assume the antibody you are referring to is tpo antibody. This is simply a marker of the disease. The level is irrelevant. Whether it goes up or down means nothing, and does not need to be checked repeatedly. You need more levothyroxine to bring your TSH level down.