Thyroid hormones may play a direct role in remyelination and repair in the adult CNS by promoting maturation of oligodendrocytes. Further, thyroid hormones have been shown to reduce oxidative stress and thus may have the capacity to prevent mitochondrial dysfunction as well. Since tri-iodothyronine (T3) is believed to mediate the most important thyroid hormone actions, liothyronine (synthetic form of T3) has the potential to induce reparative mechanisms and limit secondary neurodegeneration in MS. In mice, T3 administration has shown to help facilitate recovery from cuprizone-induced demyelination. In this study, the investigators propose to perform a phase 1 study in patients with MS to establish a tolerable dose of liothyronine, evaluate the safety of this medication, determine whether it impacts function, and evaluate if it is associated with changes in neurotrophic and/or inflammatory biomarkers in the cerebrospinal fluid (CSF).