The endocrine secretions of carcinomas can be life-threatening. Medullary thyroid carcinoma (MTC) is a rare cancer that is often associated with cortisol secretion, leading to paraneoplastic Cushing’s syndrome. Mutations of the proto-oncogene RET are driver molecular events in 70% of MTC cases. Here, we report a case of a woman, born in 1956, who was diagnosed with sporadic MTC in 2005, with subsequent relapses treated with focal treatments. In April 2019, she presented with severe and rapidly progressive paraneoplastic Cushing’s syndrome associated with lymph node, lung, liver and bone metastases. A supraclavicular lymph node biopsy revealed a somatic p.M918T (c.2753T>C) mutation in exon 16 of the RET proto-oncogene. The patient began treatment with selpercatinib in September 2019. Clinical efficacy was immediate. Chronic diarrhea disappeared within a few days. Clinical hypercorticism quickly disappeared, with quick improvements in muscle and skin conditions and fatigue. Two months after treatment initiation, urinary free cortisol normalized to 42 µg/24 h. Levels of the tumor markers carcinoembryonic antigen (CEA) and calcitonin also greatly decreased from baseline. After 34 months of treatment, selpercatinib elicits sustained clinical, biological and morphological responses. In summary, this case report illustrates the rapid and long-lasting antisecretory effect of selpercatinib associated with tumor control. As Cushing’s syndrome associated with medullary thyroid cancer is associated with poor prognosis, this case report is very encouraging. In addition, this suggests the potential benefit of molecular testing in all cases of medullary thyroid cancer.