Bruising at the site of the thyroid surgery
is very common in the days and weeks immediately following surgery, however it would be very rare 1 year out from the surgical procedure. If the area is red, warm, or inflamed this may be a sign of a skin infection, especially if you also have fever
or if the area of discoloration is spreading rapidly. If this is going on you should seek medical care quickly, as you will probably require antibiotics. Another possibility is that you have some pigmentation changes developing in the skin subsequent to the surgery. This can happen quite commonly, and is often the result of the healing process near the area of scar formation. Sometimes this pigmentation lessens over time, whereas other times it can be more permanent. Another question to ask yourself would be whether you feel a firm lump or swelling under the scar or under the discolored area near where the thyroid gland was removed. If so, this could be sign that some thyroid tumor is growing back and this should be checked out immediately. It sounds like your best step is first to show the area of concern to your primary care doctor
who will be able to determine if further workup is needed.