I am afraid the answer is Endocrine Surgeon since endocrine neoplasia differs from conventional oncology.Functional biology differes from tumour histology-
I think there is not a specific "region of interest", that's to say surgery of thyroid cancer should be carried out by any surgeon (endocrine, oncologist, etc...) with a specific training and a large experience on this matter.
Surgical treatment of Thyroid cancer comes under the domain of general surgeon. However, surgical oncologists and endocrine surgeons may also claim this as their domain.
I agree with Pasqualino but i also feel that this is a somewhat divisive question. Senthil's comment has some sense, but one could also argue that ENT surgeons are operating in the neck all the time and have a greater expertise for the more advanced, extrathyroidal cases. Overall I think the answer is that anyone can do this as long as they have sufficient training and experience, their background training and what label they wear is irrelevant. In the UK that can mean ENT, Maxillo Facial, General Surgeon, even a very few plastic surgeons, and don't forget Paediatric surgeons as well.