Background: Multiple sclerosis is a neurologic disorder characterized by recurrent episodes of demyelination and axonal injury. Myelin sheath destruction is associated with loss of oligodendrocytes responsible for creating and maintaining it. This leads to decrease conduction of action potential and damage even in early stages. Thyroid hormones (THs) play a critical role in the central nervous system (CNS) development, neuronal growth regulation and synaptogenesis. Any transient and moderate deficiency or excess of THs during critical periods of development or during adulthood can cause neurological abnormality. Subject and methods: A Case- control study was carried out on twenty five patients with clinically definite relapsing remitting multiple sclerosis (according to the Mc Donald criteria 2017) compared to twenty five healthy age and sex matched controls. We used the serum of the patients and the controls to measure the level of free triiodothyronine (FT3), free thyroxine (FT4), and thyrotropin (TSH) hormones using Automated Cobas e Immunoassay analyzers based on electrochemiluminescent technology that is the using of ruthenium complex and the measuring cell. Results : There was no statistically significant difference between the two studied groups in FT4 and TSH but there was a statistically significant increase in frequency of decreased FT3 among patients group. There was negative significant correlation between FT3 and Expanded Disability Status Scale (EDSS). Conclusion: Our study demonstrates a decrease in the level of thyroid hormone (Free T3) among MS patients and a negative significant correlation between Free T3 and EDSS. Key words: Ft3 free triiodothyronine; Ft4 free thyroxine; TSH thyrotropin