TSH, also known as thyroid stimulating hormone or thyrotropin, is a chemical compound that is produced and secreted by the pituitary gland. The pituitary gland is a small gland that is located in the center of the head. It protrudes downward from the bottom surface of the brain on a thin stalk that extends from a region of the brain called the hypothalamus. The pituitary gland is considered to be the master gland of the endocrine system. It produces and secretes a number of different hormones that regulate the function of other endocrine glands. There are special cells within the pituitary gland called thyrotropes that produce and store TSH. These thyrotropes secrete TSH in response to signals that are transmitted to the pituitary gland from the hypothalamus. TSH secreted by the pituitary gland enters the bloodstream and travels to the thyroid gland where it binds to special structures on the outer surfaces of follicular cells called TSH receptors. This binding process stimulates the thyroid gland to take up iodine that is used to produce thyroid hormone. It also stimulates the secretion of thyroid hormone from the thyroid gland. In turn, some of the thyroid hormone that is secreted by the thyroid gland travels to the hypothalamus and the pituitary gland where it acts to regulate the secretion of TSH. The feedback loop that governs the secretion of TSH and thyroid hormone is often a source of much confusion. There are different ways of visualizing the interactions between the pituitary gland and the thyroid gland that may help to explain it more clearly.
The thermostat and the furnace - Imagine that the pituitary gland is a thermostat and the thyroid gland is a furnace. If the temperature gets to be too cold, the body turns up the thermostat, driving the pituitary gland to secrete more TSH. The increased level of TSH drives the thyroid gland to secrete more thyroid hormone. The increased amount of thyroid hormone generates more heat. If the temperature gets to be too warm because there is too much thyroid hormone generating too much heat, the body automatically turns down the thermostat, decreasing the amount of TSH secreted by the pituitary gland. This decreases the amount of thyroid hormone secreted by the pituitary gland, decreasing the amount of heat that is generated.
The weight and the spring - Imagine that TSH is a spring that is standing upright, and that the thyroid hormone produced by the thyroid gland is a weight sitting on a platform on top of that spring. When there is an adequate amount of thyroid hormone on the platform, the TSH is compressed to a certain height. If there is too much thyroid hormone on the platform, the TSH becomes shorter as it is compressed downward. If there is not enough thyroid hormone on the platform, the TSH becomes taller as it expands upward.