The thyroid function is regulated by the TSH. That is produced by the pituitary. TSH is under the control of TRH produced by the hypothalamus. Thyroid hormone secretion is under control by a negative feedback loop system. T4, and T3 feed back on the TRH and TSH to inhibit them. Development of the thyroid and pituitary are needed for proper functioning system. The cells that produce TSH in the pituitary are called Thyrotropes.