Thyroid stimulating hormone (TSH) is a glycoprotein hormone secreted by the anterior pituitary gland. TSH has two subunits, namely α and β. The α subunit of TSH is similar to the α subunit found in the LH, FSH and hCG glycoprotein hormones. However, the β subunit is specific and differs from hormone to hormone. The thyroid hormones are secreted and produced by the thyroid gland. The production of thyroid hormones is under the regulation of TSH. Also, TSH acts as a stimulator of iodide transport and the gland itself is under the positive control of TSH. The concentrations of thyroid hormones control the secretion of TSH, therefore, a negative feedback exists. It is to be noted that the secretion of thyroid hormones are under the direct, positive effect of the sympathetic nervous system. The major protein component of the thyroid gland is thyroglobulin, a glycoprotein of which the secretion in the blood stream is stimulated by TSH. Therefore, TSH plays an important role in the proper function and development of the thyroid gland. It is recommended to assay both the glycoprotein hormone and the target organ hormones. For example, in primary hypothyroidism the serum level of thyroxine is low while the TSH level is high. In secondary hypothyroidism, both thyroxine and TSH are low. The TSH level is decreased in hyperthyroidism. Today, with all the sensitive assays available, if there were to be only one test to be prescribed for thyroid function, TSH would be the test. TSH determinations are also helpful to monitor patients who receive thyroxine replacement therapy.
- References to Thyroid Stimulating Hormone (TSH, Thyrotropin)