Alemtuzumab is an anti-CD52 monoclonal antibody used in the treatment of relapsing-remitting multiple sclerosis (MS). Between 20-40% of alemtuzumab-treated MS patients develop autoimmune thyroid disease (AITD) as a side effect.To determine whether MS disease progression following alemtuzumab treatment differs in patients that develop AITD compared to those who do not.A retrospective analysis of 126 patients with relapsing-remitting MS receiving alemtuzumab from 2012 to 2017 at a tertiary referral centre.Thyroid status, new relapses, Expanded Disability Status Score (EDSS) change and disability progression following alemtuzumab were evaluated.Twenty-six percent (33 out of 126, 25 female, 8 male) of alemtuzumab-treated patients developed AITD, 55% of which was Graves' disease. EDSS score following alemtuzumab was reduced in patients who developed AITD compared to those who did not (median [IQR]; AITD: -0.25 [-1 - 0.5] vs non-AITD: 0 [1 - 0]. P=0.007]. Multivariate regression analysis confirmed that the development of AITD was independently associated with EDSS score improvement (p=0.011). Moreover, AITD patients had higher relapse-free survival following alemtuzumab (p=0.023). There was no difference in the number of new focal T2-lesions and contrast-enhancing MRI lesions developed following alemtuzumab between the two groups.Graves' disease was the most common form of AITD developed by MS patients following alemtuzumab. This study suggests that MS patients who develop AITD may have an improved response to alemtuzumab, as measured by reduced disability and lower relapse rate.