The thyroid gland is designed to be in perfect harmony with the pituitary gland and the endocrine system. After a meal, or in anticipation for energy needs, the hypothalamus receives the summons to initiate the production of thyroid hormones. The hypothalamus provides a signal through the production of thyrotropin-releasing hormone (TRH) to the pituitary to produce thyroid-stimulating hormone (TSH). TSH provides a positive stimulus for the thyroid gland to produce and release T4 into circulation, where T3 is derived and imparts its action on the metabolic processes. Excess T4 and T3 in circulation provide a negative feedback to the hypothalamus and pituitary to stop signaling the need for additional thyroid hormone production. This is a classic negative feedback loop, and when in synchrony, results in a very narrow and tightly regulated concentration of available free T4 (FT4) and free T3 (FT3) in circulation.