The thyroid gland, the body's largest endocrine gland, is situated in the neck just below the thyroid cartilage and secretes hormones necessary for growth and proper metabolism. A deficiency or overabundance of the secreted hormones results in one of two endocrine disorders, hyperthyroidism and hypothyroidism.

Hyperthyroidism results from an excessive production of the thyroid hormone, thyroxine. Thyroxine production is dependent on a sufficient intake of iodine and on stimulation by thyroid-stimulating hormone (TSH) from the pituitary gland. Symptoms of hyperthyroidism include an increase metabolism, rapid pulse, weight-loss despite normal or increased appetite, irritability, and occasionally, an enlargement of the thyroid gland itself. Because the body's metabolism is increased, patients often feel warmer than those around them. An extreme overproduction of thyroid hormones leads to a dangerous condition called thyrotoxicosis.

The most common cause of hyperthyroidism is Graves' disease, an autoimmune disorder characterized by a chronic course of remissions and relapses. The condition is usually accompanied by a protrusion, or bulging, of the eyeballs (exophthalmos).

Patients with hyperthyroidism are treated with an anti-thyroid drug or radioactive iodine, which destroys some thyroid tissue and subsequently reduces hormone secretions.

Hypothyroidism occurs when the thyroid gland does not secrete enough thyroxine. Deficiencies can occur when there is insufficient iodine in the diet or a glandular malfunction. Hypothyroidism is a common disorder, with subtle symptoms that can progress for years before being recognized and treated. The condition is characterized by a low metabolism, sluggishness, weight gain, hair loss and dry skin. Patients are often colder than those around them. Treatment is by administration of thyroxine. In the past, a disease known as goiter resulted from the lack in thyroid hormone secretions, but the use of iodized salt has helped eliminate the disorder.