Swelling in the neck is the most visible form of thyroid disease. Lack of iodine in the diet affects the thyroid gland and leads to swollen necks known as goiter. Located in the neck, thyroid is an endocrine gland that produces essential hormones. Thyroxine is the main hormone produced and iodine plays a chief role in its production. It regulates the rate of metabolism and influences the growth and rate of function of other systems. Children with thyroid hormone deficiency often have stunt growth and impaired brain.

Under activity of the thyroid called hypothyroidism and over activity called hyperthyroidism are the main thyroid diseases. Tumor is another major disease affecting the thyroid.

Iodine deficiency is the main cause of goiter. Hashimoto's thyroiditis is another major thyroid disease. In this disease, the antibodies in the body fight the cells of the thyroid. It is hereditary and is found more common in women. Deficiency of thyroxine at the time of birth also leads to goiter. If not detected in the initial stage, it can lead to permanent mental retardation. Inflammation of thyroid gland is another common thyroid disease. It often develops into several other complications.

Inadequate production of thyroid hormone is the chief cause of hypothyroidism. Symptoms associated with hypothyroidism are sometimes misleading and are mistaken for other diseases. If not diagnosed early the effect of hypothyroidism can be quite serious. Stunt physical and mental growth called cretinism is a major disease caused due to hypothyroidism.

Excess production of thyroid hormone leads to hyperthyroidism. Grave's disease, a form of thyroiditis, is the most common cause of hyperthyroidism. Also referred as Graves-Basedow disease, it is found mainly in middle-aged women. Hurthle cell adenoma, or thyroid adenoma, is another thyroid disease. The Hurthle cell is a large cell and it stains pink. Plummer's disease, a form of goiter, is another commonly occurring thyroid disease.