A lack of iodine in your diet can lead to goiter, an abnormal enlargement of the thyroid gland.
The thyroid needs iodine to produce the hormones thyroxine (T4) and triiodothyronine (T3). When iodine is scarce, hormone production falls, prompting the pituitary gland to release more thyroid-stimulating hormone (TSH). Persistent TSH stimulation makes thyroid cells grow and multiply, creating the characteristic swelling in the neck.
Goiter does not always mean the thyroid is completely inactive: iodine-deficiency goiter can occur alongside hypothyroidism, and long-standing cases may develop areas that produce hormone independently. Other conditions, including Hashimoto’s disease, Graves’ disease, thyroid nodules, and tumors, can also cause goiter, but dietary iodine deficiency is the classic textbook association.
Iodized salt, introduced widely during the early 20th century, dramatically reduced iodine-deficiency goiter in many countries. Iodine deficiency during pregnancy is especially serious because inadequate thyroid hormone can impair fetal brain development and cause congenital iodine-deficiency syndrome.