In response to thyrotropin-releasing hormone (TRH) secretion by the hypothalamus, the pituitary gland secretes thyroid-stimulating hormone (TSH), which stimulates iodine trapping, thyroid hormone synthesis, and release of T3 (triiodothyronine) and T4 (thyroxine) by the thyroid gland. When dietary iodine intake is sufficient, the presence of adequate circulating T4 and T3 feeds back at the level of both the hypothalamus and pituitary, decreasing TRH and TSH production. When circulating T4 levels decrease, the pituitary increases its secretion of TSH, resulting in increased iodine trapping as well as increased production and release of both T3 and T4. Dietary iodine deficiency results in inadequate production of T4. In response to decreased blood levels of T4, the pituitary gland increases its output of TSH. Persistently elevated TSH levels may lead to hypertrophy of the thyroid gland, also known as goiter.