Pituitary glycoprotein hormones FSH (follicle-stimulating hormone), LH (luteinizing hormone) and thyrotropic hormone, TSH (thyroid-stimulating hormone) are formed by two subunits: alpha which is essentially the same for all three hormones, and beta which is responsible for their biochemical specificity. The alpha subunit is from the quantitative aspect a very important secretion of the pituitary. Part of the alpha subunit produced in the pituitary is not used for the synthesis of hypophyseal glycoprotein hormones and is secreted in a pulsed way synchronously with the pulsed LH secretion into the peripheral blood stream. Levels of the free alpha subunit in the peripheral blood depend on a number of factors in particular age, sex and in women on the stage of the menstrual cycle. The physiological roles of the free alpha subunit are only partially known. It is assumed that the most important part is the participation in the modulation of the effect of hypophyseal glycoprotein hormones at the level of their peripheral tissues. During the last few years it was revealed, surprisingly, that the level of the free alpha subunit in plasma may be used as a tumour marker as this parameter rises during some pathological conditions, in particular in adenomas of the pituitary.