Intracranial germinomas are rare germ cell tumours often found in pineal and suprasellar regions. Pituitary stalk germinomas are particularly rare and diagnostically challenging, especially with negative alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG) tumour markers.

An 18-year-old female was referred to the endocrinology clinic for investigation of primary amenorrhoea. On examination, she had intact smell with normal external female genitalia but absent pubic hair and Tanner stage 2 breast development. Her medical history is significant for previous surgically managed bilateral slipped femoral capital epiphyses at age 13. Karyotype showed normal 46,XX with no cytogenetic abnormalities. Pelvic MRI showed a rudimentary uterus for her age. Bone age was 2 standard deviations below her chronological age. Subsequent pituitary MRI demonstrated an ovoid lesion (8.6 x 8.6 x 19.2 mm) involving the pituitary stalk and inferior hypothalamus with optic chiasm enhancement but no clinical evidence of visual field deficits. Pituitary panel demonstrated panhypopituitarism affecting all axes with mildly elevated prolactin in keeping with a stalk lesion. Antidiuretic hormone (ADH) deficiency was present with copeptin <0.7 pmol/L after an 8-hour overnight water deprivation test and evidence of polyuria (4.61L on 24-hour collection). Tumour markers hCG and AFP were negative. Pituitary stalk biopsy confirmed germinoma on histopathology. Hydrocortisone, thyroxine and desmopressin replacement was commenced, and the patient was referred to medical and radiation oncology. She underwent 4 cycles of chemotherapy involving carboplatin and etoposide with complete radiographic resolution of the pituitary stalk lesion after 2 cycles and is currently awaiting adjuvant radiotherapy. She continues to have panhypopituitarism with ADH deficiency.

Tissue biopsy is important for diagnostic clarity in undifferentiated pituitary stalk lesions. Tumour marker-negative germinoma is a rare cause of pituitary stalk lesions which can lead to panhypopituitarism and ADH deficiency. These tumours appear to respond exceedingly well to chemotherapy.