Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism of almost every tissue in the body. It is a glycoprotein hormone produced by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid. Physiology
Hormone levels
TSH (with a half-life of about an hour) stimulates the thyroid gland to secrete the hormone thyroxine (T4), which has only a slight effect on metabolism. T4 is converted to triiodothyronine (T3), which is the active hormone that stimulates metabolism. About 80% of this conversion is in the liver and other organs, and 20% in the thyroid itself.TSH is secreted throughout life but particularly reaches high levels during the periods of rapid growth and development, as well as in response to stress. The hypothalamus, in the base of the brain, produces thyrotropin-releasing hormone (TRH). TRH stimulates the anterior pituitary gland to produce TSH. Somatostatin is also produced by the hypothalamus, and has an opposite effect on the pituitary production of TSH, decreasing or inhibiting its release. The concentration of thyroid hormones (T3 and T4) in the blood regulates the pituitary release of TSH; when  T3 and T4 concentrations are low, the production of TSH is increased, and, conversely, when T3 and T4 concentrations are high,  TSH production is decreased. This is an example of a negative feedback loop.
The same TSH concentration may have a different meaning whether it is used for diagnosis of thyroid dysfunction or for monitoring of substitution therapy with levothyroxine. Reasons for this lack of generalisation are Simpsons paradox and the fact that the TSH-T3 shunt is disrupted in treated hypothyroidism, so that the shape of the relation between free T4 and TSH concentration is distorted. Therapeutic
A synthetic drug called recombinant human TSH alpha (rhTSHα or simply rhTSH) or thyrotropin alfa (INN) is manufactured by Genzyme Corp under the trade name Thyrogen. It is used to manipulate endocrine function of thyroid-derived cells, as part of the diagnosis and treatment of thyroid cancer. History
In 1916, Bennett M. Allen and Philip E. Smith found that the pituitary contained a thyrotropic substance. References
External links

"Thyrotropin alfa". Drug Information Portal. U.S. National Library of Medicine. TSH at Lab Tests Online
MedlinePlus Encyclopedia: 003684
Thyrotropin at the US National Library of Medicine Medical Subject Headings (MeSH)