Fibromyalgia is a disorder characterised by fatigue, generalised body pain and cognitive symptoms, and the aetiology has not been clearly described. Because similar symptoms are frequently found in Hashimotos thyroiditis and there is not an optimal response to the treatment of hypothyroidism, the aim of this study is to determine the prevalence of fibromyalgia in patients with Hashimotos thyroiditis, to explain the symptoms and to select the appropriate treatment. This cross-sectional study used the case study method and included 100 euthyroid patients with Hashimotos thyroiditis that were referred to the rheumatology clinic of Imam Khomeini hospital in 1392. Patients who met the inclusion criteria underwent a physical examination according to ACR 2010 criteria for fibromyalgia. Data collection and analysis was performed using SPSS 22 software. The mean age of the patients was 38.7±11.58 years, and 7% of patients were male. The prevalence of fibromyalgia was 5%. No significant correlation was found between fibromyalgia patients and other variables such as age, gender, marital status, menopause, education, duration of Hashimotos thyroiditis, Anti-TPO and TSH levels, and also no correlation was found between the severity of fibromyalgia and these variables. The prevalence of fibromyalgia was not higher than the normal population; thus, musculoskeletal symptoms could not be justified only with fibromyalgia. According to the low number of samples and lack of information on the prevalence of the disease in Iran, further studies using a larger population are recommended.