Free Alpha Subunit of hCG

The alpha subunit is a 91 amino acid protein chain bound non-covalently to specific beta chains to form the glycoproteins lutenising hormone (LH), follicle stimulating hormone (FSH), thyroid stimulating hormone (TSH) and human chorionic gonadotrophin (hCG). The alpha subunit is produced by a different gene to the beta subunits and its production is under a different control mechanism. As the binding is non-covalent free (unbound) alpha subunit is present in the circulation in direct relationship to the amount of the intact hormones.

As the levels of free alpha subunit are related to the other hormone levels, increased concentrations are seen in the circulation in pregnancy (very high levels), post menopause (increased LH and FSH) and in hypothyroidism with increased TSH.

The clinical use of free alpha subunit is as a tumour marker for pituitary tumours in cases where other hormones are not secreted. A role is also proposed to differentiate a TSH secreting adenoma from thyroid hormone resistance, two conditions where TSH may be innappropriately high for the level of thyroid hormones in the circualtion. 

Free alpha subunit is measured at Sydpath with the Beckman Coulter Immunoradiometric assay (IRMA). The assay is tracaeble to the international refernce preparation NIBSC 75/569. Results are reported in IU/L (International Units per Liter). Results can be converted to mass units (ug/L) as follows: 1 IU/L = 1 ug/L.