Earlier work suggested that the pituitary of fish synthesizes a single gonadotropin, called maturational gonadotropin, which regulates all processes of reproduction. However, the existence of two gonadotropins was established in a salmonid species with the discovery of a new gonadotropin, called Fsh (formerly GTH-I), chemically different from the previously known maturational gonadotropin, or Lh (formerly GTH-II). Lh and Fsh were shown to be heterodimeric glycoproteins hormones, each consisting of a common a and hormone-specific p subunit which are noncovalently linked.
The amino acid sequence of the Lhb subunit is highly conserved, particularly in regions thought to be important for receptor interaction, while the primary structure of the Fshb subunit is more variable than that of the Lhb subunit even in regions thought to confer ligand specificity. Further sequence analysis demonstrated that fish GTH-I and GTH-II were homologous to tetrapod FSH and LH, respectively. This homology was supported by physiological studies showing that Fsh (formerly GTH-I) levels are higher at the beginning of the reproductive cycle, whereas Lh (formerly GTH-II) increases later in the cycle.