Functions as a heterodimeric glycoprotein hormone with GPHB5 able to bind and activate the thyroid-stimulating hormone receptor (TSHR), leading to increased cAMP production. Plays a central role in controlling thyroid cell metabolism. Heterodimer with GPHB5; this heterodimer interacts with thyroid-stimulating hormone receptor (TSHR), and hence stimulates cAMP production. Belongs to the glycoprotein hormones subunit alpha family.