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). Heterodimer with GPHB5; this heterodimer interacts with thyroid-stimulating hormone receptor (TSHR), and hence stimulates cAMP production. Found in a variety of tissues. Glycosylated. Belongs to the glycoprotein hormones subunit alpha family.