Iodine deficiency primarily affects the thyroid gland because iodine is an essential component of thyroid hormones (T3 and T4). Without adequate iodine, the thyroid cannot synthesize sufficient hormones, leading to compensatory glandular hyperplasia or goiter. This deficiency disrupts metabolic regulation and can cause hypothyroidism. The pituitary gland responds by increasing TSH secretion to stimulate the thyroid, but the deficiency prevents normal hormone production. Thus, the clinical impact of iodine deficiency is directly related to the thyroid’s hormone synthesis function.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.