An enlarged thyroid gland can be caused by an iodine deficiency, Grave's disease or Hashimoto's disease, according to the Mayo Clinic. Other possible causes include thyroid nodules, pregnancy and thyroid cancer.
The abnormal enlargement of the thyroid gland is called a goiter. However, having a goiter does not always indicate a problem or malfunction of the thyroid gland, according to the American Thyroid Association. A goiter can result in response to hyperthyroidism and hypothyroidism, and even in euthyoidism. Hyperthyroidism refers to the overproduction of thyroid hormones, while hypothyroidism refers to an underproduction. With euthyroidism, the thyroid gland secretes normal amounts of thyroid hormone.