Endocrine Abstracts

Case: A 52-year-old gentleman was admitted with dizziness and collapse. Past medical history revealed dyslipidemia treated with statin. Physical examination was unremarkable. Initial investigations were consistent with SIADH (sodium: 109 mmol/l, serum osmolality: 243 mOsm/kg, urine Na: 59 mmol/l and urine osmolality: 191 mOsm/kg). Urea, creatinine and random glucose were normal. CT Head showed pituitary enlargement with sellar expansion. Pituitary profile showed high prolactin: 8305 mIU/ml, low IGF-1: 78.1 μg/l and low testosterone: 3.83 nmol/l with normal LH and FSH. Thyroid function was normal with intact pituitary–adrenal axis (normal Glucagon and ACTH stimulation tests). MRI pituitary confirmed macroadenoma with heterogeneous solid-cystic mass (2.0×1.9×1.5 cm) with deviation of pituitary stalk. He was treated with Cabergoline 0.5 mg per week and few days of fluid restriction with improvement of sodium level to 137 mmol/l and prolactin to 61 mIU/l after 2 weeks. Over 5 months follow up; he remained asymptomatic with normal sodium and prolactin on Cabergoline. Hypogonadisim has resolved and IGF-1 has normalized. Repeat MRI 4 months after discharge showed significant reduction in macroadenoma size to 1.4×1.3×0.7 cm with pituitary stalk being in midline.

Discussion: Hyponataemia complicating pituitary macroadenomas is usually related to secondary hypoadrenalism. However, the presence of SIADH with intact pituitary–adrenal axis in such cases is very rare with only seven cases reported in literature (six non-functioning macroadenomas and one macroprolactinoma). The mechanism of AVP release in these cases is not fully understood, but is thought to be related to local mechanical stress on the AVP neurons axonal terminal and dislocation of pituitary stalk and neurohypophysis by pituitary tumor.

Conclusion: We hereby report the second case of SIADH secondary to macroprolactinoma with normal pituitary–adrenal axis. Pituitary macroadenomas should be considered in differential diagnosis of SIADH even in context of preserved pituitary-adrenal axis.