Shared alpha chain of the active heterodimeric glycoprotein hormones thyrotropin/thyroid stimulating hormone/TSH, lutropin/luteinizing hormone/LH and follitropin/follicle stimulating hormone/FSH. These hormones bind specific receptors on target cells that in turn activate downstream signaling pathways. Heterodimer. The active hormones thyrotropin, lutropin and follitropin are heterodimers composed of CGA, a common alpha chain described here and a unique beta chain which confers their biological specificity to the hormones: TSHB for thyrotropin, LHB for lutropin and FSHB for follitropin. Belongs to the glycoprotein hormones subunit alpha family.