Dear Doctors, Today I wish to share a case of PUO and the diagnostic steps to reach the cause of puo. Our patient had a rare cause of hashimoto's thyroidits presenting as a PUO. A 60 yr old female patient, hailing from dombivli area, post grad in pathology presented to my OPD with history of multiple episodes of fever since august 2022. Fever was low grade without chills, evening rise and a 1 kg weight loss. Each episode of fever lasted for few days and improved with crocin and some antibiotics. Occasional sore throat, body pain and weakness was present. The episodes kept coming every month for 3 months. Patient being a post grad in pathology had done a full fever profile which was negative. Cbc was unremarkable, malaria, dengue, lepto, tuberculosis, salmonella, brucella, urine exam were all negative. Hep B, Hep c, HIV elisa were negative. Usg abdomen, xray chest were unremarkable. Blood culture negative. She then visited me to know the cause of fever and a solution to it. A full body pet ct was done which showed an increase uptake in thyroid area. No other abnormalities were seen on pet scan. We then got her T3, T4, TSH and anti TPO antibody levels. TSH and anti tpo antibody were raised suggesting hashimotos thyroidtis as the diagnosis. I started her on thyroid supplementation as per her body weight and today is a month since then. She now feels better and has not had any fever episodes post supplementation. She also is happy that she was treated on an OPD basis without any indoor admission. On literature search I got to know hashimotos thyroiditis is a rare cause of PUO. If we follow a step by step process of investigation one can reach the cause of PUO. Dr Hardik Thakker, MD MED, ECFMG (USA).