Thyroid Hormone Production An elegant system: The thyroid regulates its hormone output with the aid of the hypothalamus and the pituitary gland. They hypothalamus secretes thyrotropin-releasing hormone (TRH), triggering the pituitary to make thyroid stimulating hormone (TSH). TSH tells the thyroid to capture iodine from the blood to synthsize, store, and releases thyroxin (T4). When T4 reaches target cells, it is converted to triiodothyronine (T3). They hypothalamus and pituitary reduce their output of TRH and TSH once T4 reaches an adequate circulating level, then resume output when the T4 level again drops.