Alpha-subunit measurement is used for the screening of anterior pituitary where there is evidence of a disease that may be due to a pituitary adenoma.
It can also be used in the control of hypothyroidism and hyperthyroidism with inadequate TSH secretion, to differentiate pituitary resistance to thyroid hormone from thyroid adenomas. It is also used to monitor the treatment of thyroid adenomas.
Measurement of α-subunit is required to investigate pituitary gonadotropin adenomas and in particular the so-called non-functional pituitary adenomas. This is useful since secretion of α-subunit is not associated with specific endocrine symptoms as the α-subunit has no characteristic biological activity.
Along with human chorionic gonadotropin (hCG) the three pituitary glycoprotein hormones LH, FSH and TSH are composed of two subunits, the α- and β-subunits, linked non-covalently. Though β-subunit is specific to each hormone, all 4 hormones have the same common α-subunit. The free subunits are not biologically active, while α- and β- subunits should be linked together in order to express their specific and special hormonal activity.
The α-subunit is identical for all three pituitary hormones and the chorionic gonadotropin. It consists of 92 amino acids and is encoded by a single gene. The α-subunit is secreted by impulses from anterior pituitary cells. Under normal conditions, its secretion rate coincides with LH fluctuations, indicating that LHRH plays a role in regulating its secretion. Physiological variants and pharmacological stimulation or inhibition of pituitary hormones are accompanied by parallel changes in the α-subunit. There is, however, one exception to this rule, which is observed during long-term treatment with LHRH agonists, where α-subunit concentrations remain high, while LH and FSH concentrations are very low.
The α-subunit does not appear to have any biological activity.
Possible Interpretations of Pathological Values
- Increase: Gonadotropic adenoma, prolactin adenoma, dysfunctional adenoma, primary thyroid adenoma, choriocarcinoma, other malignant diseases such as pancreatic cancer, liver and gastrointestinal as well as benign gastrointestinal disease and administration of GnRH or similar drugs.