Thyroid-stimulating hormone (TSH) is a glycoprotein comprised of two noncovalently linked subunits, or and p. The structure of TSH from a variety of species has been elucidated, including the amino acid sequence and carbohydrate composition.'Y2 Bovine TSH has been particularly well characterized and its a subunit has a molecular weight of 14,000, of which 11,000 is composed of a protein core of 96 amino acids and 3,000 represents two oligosaccharide units linked to asparagine residues. Bovine TSHP has a molecular weight of 15,000 of which 13,000 is comprised of a protein core of 113 amino acids and 2,000 represents one asparagine-linked oligosaccharide unit. TSH is structurally related to the pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well to the placental hormone chorionic gonadotropin (CG). Within a single species the a subunits from each of these glycoprotein hormones are virtually identical, whereas the p subunits are unique and confer immunologic, receptorbinding, and biologic specificity. Attainment of the conformation necessary for hormonal activity is dependent on proper assembly and carbohydrate processing of the TSH subunits, whereas the free subunits are essentially devoid of receptor binding and biologic activity. 2