Alemtuzumab is a humanized monoclonal antibody approved for the treatment of adult patients with active multiple sclerosis (MS) in more than 65 countries [
1,
2]. In clinical trials, alemtuzumab demonstrated superior efficacy compared to an active comparator (subcutaneous interferon beta-1a) in both treatment-naive patients and in those with inadequate response to prior therapy [
3‐
5]. The mechanism by which alemtuzumab exerts its therapeutic effects in MS is not fully elucidated. However, research suggests immunomodulatory effects through the depletion of mature and circulating B- and T lymphocytes, followed by a repopulation and long-lasting shift in immunological balance, including alterations in the number, proportions, and properties of some lymphocyte subsets, increased representation of regulatory T-lymphocyte subsets, and increased representation of memory T- and B lymphocytes. This mechanism may be relevant to the durable efficacy of this drug [
6‐
8]. Treatment may result in the formation of autoantibodies and increase the risk of autoimmune mediated adverse events. Autoimmune thyroid disease (AITD) has been the most common autoimmune disorder, followed by immune thrombocytopenia (ITP), and nephropathies. According to a hypothesis, differential kinetics of lymphocyte subset reconstitution causing early hyperrepopulation of immature B lymphocytes may explain the occurrence of autoimmune events [
9]. A risk management plan (RMP) has been implemented to mitigate the risk of autoimmune conditions in MS patients treated with alemtuzumab. Thyroid function tests, such as serum thyroid stimulating hormone (TSH) levels, should be obtained prior to initiation of treatment and every 3 months thereafter until 48 months following the last infusion. After this period, testing should be performed based on the clinical findings suggestive of thyroid dysfunction. Systematic thyroid function monitoring was integrated as of the phase 2 clinical trial [
3]. In the phase 3 clinical trials and the extension study, 40.7% (CARE-MS I) and 37.7% (CARE-MS II) of alemtuzumab-treated subjects developed thyroid adverse events (TAEs) within 5 years, more precisely thyroid dysfunction, with a peak incidence in the third year following the first administration [
10‐
13]. In the event of abnormal findings, the treating physician will have to decide how to manage the sequence of additional tests, possible treatments, and the need to refer the patient to an endocrinologist for further treatment and follow-up. Post-marketing surveillance figures have not been published so far. Due to the current lack of practical clinical guidance with regard to managing thyroid-related autoimmunity after alemtuzumab treatment, a Belgian taskforce was created in 2016 consisting of experts in MS and in thyroid disease, with the objective of issuing practical recommendations to guide the treating physician.