GPHA2 is a member of the glycoprotein hormones subunit alpha family. Glycoprotein hormones consist of two subunits, the common alpha- and specific beta-subunits, which associate noncovalently to form a heterodimer. The alpha-subunit combines with four distinct beta-subunits giving rise to four biologically active hormones in the human: FSH, LH, TSH, and CG. GPHA2 and glycoprotein hormone beta 5 (GPHB5) can form a noncovalent heterodimer. GPHA2 can be detected in a variety of tissues. Recombinant A2/B5 heterodimeric glycoproteins activate human TSH receptors, but not LH and FSH receptors, and shows high affinity to TSH receptors in a radioligand receptor assay. The heterodimer also stimulates cAMP production and thymidine incorporation by cultured thyroid cells and increases serum thyroxine levels in TSH-suppressed rats in vivo. This new heterodimeric glycoprotein hormone was named Thyrostimulin based on its thyroid-stimulating activity. The expression of Thyrostimulin in the anterior pituitary known to express TSH receptors suggested a paracrine mechanism.