Abstract Disclosure: P. Finny: None. T. George: None. A.K. Francis: None. Background: Parkinsonism is a rare neurological manifestation of primary hyperparathyroidism. Radiofrequency ablation (RFA) of parathyroid adenomas is a novel treatment modality. Clinical Case: A 78 year old lady presented to our Emergency department with hypercalcemic encephalopathy. She had long standing Diabetes mellitus, Bronchial asthma and Parkinsonism with dementia for the past 14 years. She was bed bound with dependence on carers for activities of daily living (ADL). She had been diagnosed elsewhere in 2018 to have primary hyperparathyroidism due to a right inferior parathyroid adenoma and had a right hemithyroidectomy with a failed parathyroidectomy. She was being managed with Cinacalcet 60 mg daily for the past 4 years. Prior to this admission she had discontinued Cinacalcet for a few days. Her initial serum calcium was 15.3 mg/dl (8.7-10.7), phosphorus 1.80 mg/dl (2.5-4.5) and PTH 451 pg/ml (15- 88). She was aggressively treated with intravenous normal saline hydration followed by Furosemide injection. Then she was given Calcitonin injections subcutaneously and IV Zoledronic acid infusion. Her calcium dropped to 11.8 mg/dl within 48 hours of admission, and her sensorium improved. Imaging studies showed no visible parathyroid adenoma on ultrasound, but the 4D CT scan of the neck revealed a left inferior parathyroid adenoma of size 1.1 x 1.2 cm. This was concordant on the Technetium 99m Sestamibi scan. Her MRI Brain showed bilateral globus pallidus calcification. In view of her multiple comorbidities making her a poor candidate for general anaesthesia, non-surgical options were considered. A percutaneous ultrasound guided RFA of the left parathyroid adenoma was successfully performed. Post procedure the PTH dropped from 321 pg/ml to 54.9 pg/ml within 24 hours. The calcium, phosphorus and PTH remained normal. Her Parkinsonism and dementia significantly improved and we could wean her off the anti- parkinsonian and anti-psychotic medication. She became ambulant and regained her cognitive functions. This clinical and biochemical remission was maintained over the next 22 months of follow up. Conclusion: RFA is a novel therapeutic option in treating parathyroid adenomas among surgically unfit patients. Reversal of advanced Parkinsonism following correction of chronic hypercalcemia led to this patient regaining a good quality of life. 4D CT scan of the neck is useful in localizing the parathyroid adenoma when there is discordance between the ultrasound and Sestamibi scan.