Have you had blood work done to see what your thyroid hormone levels are? You should have, at least, TSH, Free T3 and Free T4. Also, get tested for thyroid antibodies to determine if you have an autoimmune thyroid disease, and get a thyroid ultrasound to check for nodules, etc.
What, if any, other symptoms do you have?
Looks a lot like mine, except mine is more prominent on the right, and less central like yours. Probably a good idea to make a trip to see your doctor. How long have you noticed that?