TSH

Thyroid-stimulating hormone or TSH is a naturally occurring hormone produced by the pituitary that stimulates the production of thyroid hormones. TSH is, in turn, regulated by the release of TRH (which increases TSH) and Somatostatin (which decreases TSH) by the hypothalmus. 
Doctors will usually test TSH levels to see if they are within normal range. Values outside normal range may show a patient is at risk for developing thyroid conditions. Doctors will also check the ratio between TSH and thyroid hormones. This can help pinpoint whether the thyroid or the pituitary is the cause of many metabolic diseases.
In patients beign treated with thyroid medications, doctors will be more likely to monitor TSH levels rather than thyroide hormone levels as this level is actually to be more likely to indicate whether the thyroid is working properly. 
Recently, synthetic TSH has been developed and is used in the treatment of thyroid cancer.
Thyroid-stimulating hormone at Wikipedia
Category:Endocrinology