Goiters can occur when the thyroid gland produces either too much thyroid hormone (hyperthyroidism) or not enough (hypothyroidism). Much more rarely, the problem may arise when the pituitary gland stimulates thyroid growth to boost production of the hormone. Enlargement could also occur with normal production of thyroid hormone, such as a nontoxic multinodular gland.
Another type of thyroid growth, called a sporadic goiter, can form if your diet includes too many goiter-promoting foods, such as soybeans, rutabagas, cabbage, peaches, peanuts, and spinach. Keep in mind you would have to eat huge amounts of these foods to cause a goiter.
These foods can suppress the manufacture of thyroid hormone by interfering with your thyroid's ability to process iodide. Historically, the most common cause used to be a lack of iodide in the diet, however in the 1920s iodized salt was introduced in the U.S. now making this a rare cause of goiters. Iodine is added to other foods as well.