Thesis advisor: Sarah McMenamin / Pectoral fins are evolutionarily homologous to tetrapod limbs and can serve as useful models for studying the genetic and hormonal factors regulating appendage development. To test the roles of thyroid hormone (TH) in the development of these appendages, we examined the morphogenesis of zebrafish pectoral fins under euthyroid (EuTH) or hypothyroid (hypoTH) conditions. TH plays an important role in modulating the development of the pectoral fin endoskeleton, as well as the proximo-distal patterning of the fin rays. Additionally, in HypoTH fish, shh was expressed in the same domains, but at ~50% of WT expression levels. Nuclear TH acts by binding to dual-action receptors, including Thrab, which represses or activates expression depending on interaction with the TH ligand. When Thrab was absent in HypoTH fish, we found that many elements of the HypoTH phenotype were rescued, suggesting TH relieves Thrab- mediated repression. We also found that TH modulates the development of the musculature surrounding the pectoral fin. Lastly, we generated CRISPR-Mediated knockouts of dio1 and dio3b, to learn how the deiodination of TH may be affecting the development of the pectoral fin. / Thesis (BS) — Boston College, 2023. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Biology.