‘The thyroid gland’ illustrates how the thyroid takes up iodine from the blood and stores it in a concentrated form attached to a protein, thyroglobulin. When stimulated by a pituitary gland hormone, thyroid cells build up thyroglobulin but also turn some of it into the hormones thyroxine and triiodothyronine. These hormones speed up a cell’s use of oxygen and so accelerate metabolism in most tissues and organs. This increased metabolism extends to processes like immunity, growth, and reproduction, which is why the right level of thyroid activity is important for development and health. Thyroid diseases such as hyperthyroidism and goitre, are relatively common medical conditions, but are also easily detected and treated.