Endocrine Abstracts

A 74-year- old gentleman was found to be hyponatraemic when he returned from birds watch trip with cellulitis of lower legs. He was generally fit and well apart from hypertension for which he had been on Amlodipine and Irbesartan. He was admitted with severe hyponatremia in July 2019 and was diagnosed with idiopathic SIADH as evidence of low serum osmolality (252 mosm/kg), high urine sodium (41mmol/l) and high urine osmolality (657mmol/kg). His Amlodipine and Irbesartan were replaced with Doxazosin. His thyroid function and adrenal function were normal. Underlying causes such as malignancy were investigated by CT Head and CT CAP showing no apparent causes. His sodium was closely monitored in Ambulatory Clinic for 2 months by liaising with endocrine specialist. Demeclocycline was initiated as he struggled with fluid restriction and the dose was adjusted later. He was seen in the endocrine clinic for further management in view of poor response to Demeclocycline. He was unfortunately re-admitted under urology team with new presentation of urinary retention, haematuria, hesitancy and frequency. Per-rectal examination showed moderately enlarged prostate. He was treated as an obstructive uropathy with immediate catheterization. CT abdomen and pelvis showed locally invasive prostate cancer causing left-sided hydronephrosis and hydroureter, with abdominopelvic lymphadenopathy and liver metastases. Urgent urology intervention was required to relieve obstructive uropathy. There was an unclear diagnosis between prostate cancer and rectal malignancy initially due to presence of liver metastasis and normal PSA level. Hence, MRI pelvis demonstrated the high clinical suspicious of prostate cancer with liver and bone metastasis. Flexible sigmoidoscopy and rectal biopsy excluded the rectal cancer. Giving that, normal PSA level, CT scan result and unknown cause of SIADH, Prostate biopsy was performed and confirmed the diagnosis of NET neuroendocrine tumour of prostate by showing the extensive infiltration of small cell neuroendocrine carcinoma comprising 95-100% of tumour burden. Immunohistochemistry revealed diffuse CD56 and synaptophysin positivity. There was diffuse TTF1 staining and negative PSA expression. Gut hormones profile was within normal limit.

Discussion: This case highlights the importance of investigating the underlying cause of malignancy in newly diagnosed SIADH without obvious cause. Moreover, normal level PSA does not exclude neuroendocrine tumour of prostate. Therefore, close monitoring and observation for new SIADH might reveal the underlying cause and avoid missing evolving malignancy?!