The prevalence of morbid obesity has risen to global epidemic proportions. Bariatric surgery has been proven to be a safe and effective treatment for obesity with weight reduction, resolution of obesity-related co-morbidities, improved quality of life and an increased life expectancy. Sleeve gastrectomy (SG) is the most commonly performed bariatric procedure worldwide. Obesity is associated with micronutrient deficiencies that results in the high prevalence of deficient vitamins status prior to bariatric surgery. After bariatric surgery, these micronutrient and vitamins deficiencies increase or occur de novo, and they may be threatening when left unattended. This presented clinical case demonstrates the clinical features of thiamine deficiency as well as the principles of laboratory and instrumental diagnostics. Electromyo­graphy is an informative method for diagnostic of muscle weakness. The lower serum thiamine level, neurological symptoms and electromyography results are the most important for the diagnosis of thiamine deficiency. Pathogenetic treatment can improve the patient’s condition in a short period of time. Although the SG is a purely restrictive procedure with no malabsorptive component, microelement or vitamin deficiency can occur in patients after procedure. The presented clinical case demonstrates the importance of timely thorough diagnosis and correct treatment of thiamine deficiency in patients after SG. Thus, patients after the LRS in deficiency of vitamin B1 may cause the development of neurological comp­lica­tions of varying severity, in particular dysmetabolic polyneuropathy. This condition is reversed and administration of an adequate and timely substitution therapy in patients with thiamine insufficiency allows be avoided the development of fatal complications.