What is TSH?

TSH, formally known as thyroid stimulating hormone or thyrotropin, is a hormone that is produced by the pituitary gland. The pituitary gland is a small gland that is located in the center of the head, just beneath the bottom of the brain. It is often referred to as the "master gland" of the endocrine system, as it produces a number of different hormones that act in turn to control the function of many of the other endocrine glands. TSH produced by the pituitary gland enters the bloodstream and travels to the thyroid gland where it binds to special structures called receptors. This binding process stimulates the thyroid gland to take up iodine to produce thyroid hormone which is then released into the bloodstream. In turn, thyroid hormone in the bloodstream travels to the pituitary gland where it acts to regulate the release of TSH.

This feedback loop between TSH and thyroid hormone can be somewhat difficult to visualize. There are some examples that may help to explain it more clearly.

Thermostat and Furnace
Pretend that the pituitary gland is a thermostat and the thyroid gland is a furnace. If it gets to be too cold, the body turns up the thermostat, causing the pituitary gland to produce more TSH. This stimulates the thyroid gland to produce more thyroid hormone, which generates more heat. If it gets to be too warm because there is too much thyroid hormone, the body automatically turns down the thermostat, decreasing the amount of TSH produced by the pituitary gland which in turn decreases the amount of thyroid hormone produced by the pituitary gland.

The Weight-bearing Spring
Imagine that TSH is a coil or spring that is standing upright and that thyroid hormone is a weight that sits on top of the spring. When there is an adequate amount of thyroid hormone present, the TSH is compressed to its normal length. If there is too much thyroid hormone present, the TSH is pressed down and becomes smaller. If there is not enough thyroid hormone present, the TSH expands and becomes larger.

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