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March 29, 2026Open Access

Severe Hyponatremia From Syndrome of Inappropriate Antidiuretic Hormone Secretion as the Initial Presentation of a Gonadotroph Pituitary Neuroendocrine Tumor: A Case Report

Key Points

To describe an uncommon presentation of severe hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion from a gonadotroph pituitary neuroendocrine tumor.
Reviewed clinical history and laboratory findings of a 35-year-old woman
Conducted magnetic resonance imaging (MRI) of the brain
Performed endoscopic transsphenoidal resection of the tumor
Performed histopathological analysis of the tumor tissue
Patient presented with severe hyponatremia (Na: 121 mmol/L) and low serum osmolality (252 mOsm/kg)
MRI revealed a sellar mass with suprasellar extension and optic chiasm compression
Histopathology confirmed a plurihormonal pituitary neuroendocrine tumor of gonadotroph lineage with a Ki-67 index of 10%
Endocrine assessment indicated hypogonadotropic hypogonadism with low levels of key hormones

Abstract

Pituitary neuroendocrine tumors, formerly referred to as pituitary adenomas, are common intracranial neoplasms. Most are benign and slow-growing. Clinical manifestations typically arise from hormonal hypersecretion or mass effect on adjacent structures. Hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion is an uncommon presentation of pituitary tumors and has been rarely reported. We present the case of a 35-year-old woman with a history of secondary amenorrhea and infertility who exhibited progressive headache and recurrent presyncope episodes. Laboratory tests showed that the patient had severe hyponatremia (sodium Na: 121 mmol/L), low serum osmolality (252 mOsm/kg), inappropriately elevated urine osmolality (748 mOsm/kg), elevated urinary sodium (112 mEq/L), hypouricemia (1. 9 mg/dL), and a fractional excretion of uric acid of 11. 66%, consistent with syndrome of inappropriate antidiuretic hormone secretion. Hormonal assessment indicated hypogonadotropic hypogonadism characterized by diminished levels of follicle-stimulating hormone, luteinizing hormone, and estradiol. Magnetic resonance imaging of the brain showed a sellar mass measuring 19 × 31 × 32 mm with a suprasellar extension and compression of the optic chiasm. Visual field testing corroborated bitemporal hemianopsia. The patient had an endoscopic transsphenoidal resection of the lesion. Histopathological examination identified a plurihormonal mature pituitary neuroendocrine tumor of gonadotroph lineage, exhibiting positive immunohistochemical staining for SF-1 and INSM1, negative expression for Pit-1, and a Ki-67 proliferation index of 10%. This case illustrates an uncommon manifestation of a gonadotroph pituitary neuroendocrine tumor presenting as severe hyponatremia secondary to syndrome of inappropriate antidiuretic hormone secretion. Identifying this unusual presentation is crucial, as prompt diagnosis and intervention may avert considerable neurological, metabolic, and visual complications.

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