Thyrotropin (TSH), human follicle stimulating hormone (FSH), human luteinizing hormone (LH) and human chorionic gonadotropin (hCG) are glycoprotein hormones. They are composed of two distinct, non-covalently bound subunits: a and ß. The a-subunits of TSH, LH, FSH and hCG are immunologically identical. Certain amount of free a-subunit is found under normal conditions in the circulation. Its measurement is useful in the diagnosis and follow-up of pituitary tumors and trophoblastic diseases.