Functions as a heterodimeric glycoprotein hormone with GPHB5 able to bind and activate the thyroid-stimulating hormone receptor (TSHR), leading to increased cAMP production (PubMed:12045258). Plays a central role in controlling thyroid cell metabolism (PubMed:12045258). . This is the function of ENSG00000149735 (GPHA2, glycoprotein hormone subunit alpha 2).