A paraneoplastic neurological syndrome (PNS) is a rare complication in patients with cancer and may be associated with the presence of onconeural antibodies. The anti-Hu antibodies are well characterised and hence underlying neoplasia should be rule out. Thyroid cancer may appear as metastatic disease of an unknown primary origin.

A 62-year old woman presented with a palpable left laterocervical adenopathy. A neck, chest, abdomen and pelvis CT scan showed laterocervical and retroperitoneal lymph-node metastasis. No thyroid lesions were found in neck ultrasound and a surgical excision of left laterocervical adenopathy was performed. Biopsy and immunohistochemical staining results described a poorly differentiated carcinoma vimentin, PAX8 and TTF1 positive. A 18-FDG PET was performed and no lymph-node metastasis were found hence clinical observation was decided. After one year of follow-up, the patient developed a sensory peripheral neuronopathy. A complete examination, EMG and blood tests were performed. Paraneoplastic antibodies were tested and anti-Hu antibody reactivity was positive. A neck ultrasound was repeated which revealed two moderately suspicious nodules requiring FNA biopsy per the ACR-TIRADS classification. The patologic findings described a remarkable chronic thyroiditis and a folicular lesion of undetermined significance.

The case was discussed in a multidisciplinary team and a total thyroidectomy was scheduled. There were no postoperative complications. The definitive anatomopathologic report described a multifocal papillary thyroid carcinoma BRAF positive.

We report the first case of anti-Hu antibody-positive paraneoplastic sensory neuronopathy in a patient with papillary thyroid carcinoma that initially presented as a cancer of unknown primary origin.