Depends on cause. A goiter, or enlarged thyroid, may be associated with either hypothyroidism (hashimoto's disease, iodine deficient), or hyperthyroidism (toxic nodules, graves' disease) or euthyroidism (any of above, but with normal thyroid function). Treat low thyroid with thyroid hormone (or iodine in deficient regions). Can treat high thyroid with i-131. Or surgery if thyroid gland is too large in all cases.
Neck enlargement +. Goiters occur when there is thyroid deficinecy. The thyroid sits in the middle of you neck over the wind pipe. Goiters occure when there is thyroid defieciency and the thyroid is trying to produce enough active thyroid hormone to meet your need. The thyroid hypertrophies (enlarges). Thyroid/iodine replacement and surgery may be needed to treat. An endocrinologist is the specialist usually.