The thyroid gland plays an important role in tissue metabolism and development. It secretes thyroxine (3,5,3’5′-tetraiodothyronine), which is abbreviated as T4, and small amounts of 3,5,3′-triiodothyronine, abbreviated T3. Both have systemic effects. Abnormal thyroid hormone levels lead to hypothyroid and hyperthyroid states. Inadequate thyroid hormone during development leads to congenital hypothyroidism (also known as cretinism) with associated irreversible brain damage.