Hiponatremia didefinisikan sebagai kadar natrium serum <135 mEq/L. Hiponatremia sering dijumpai, tetapi etiologinya sulit dipastikan. Seorang laki-laki berusia 64 tahun datang dengan keluhan lemas sejak beberapa hari. Pada pemeriksaan laboratorium ditemukan hiponatremia, hipoosmolalitas serum, peningkatan ekskresi natrium urin, serta peningkatan osmolalitas urin; sesuai dengan karakteristik syndrome of inappropriate antidiuretic hormone (SIADH). Pemeriksaan hormonal serum dan MRI kepala menemukan prolaktinoma dan penurunan kortisol serum yang menandakan kondisi insufisiensi adrenal. Terapi infus NaCl dan kapsul NaCl serta hydrocortisone berhasil memperbaiki keadaan pasien, baik secara klinis maupun laboratorium. Hyponatremia is defined as a serum sodium level of <135 mEq/L. Hyponatremia is a common finding but its etiology is often difffcult to be determined. A 64-year-old male came with lethargy since a few days before being admitted to the hospital. Laboratory examination revealed hyponatremia, serum hypoosmolality, increased urinary sodium excretion, and increased urine osmolality, are in accordance with the characteristics of syndrome of inappropriate antidiuretic hormone (SIADH). Serum hormonal examination and head MRI revealed a prolactinoma and decreased serum cortisol which indicated an adrenal insufficiency. NaCl infusion and oral NaCl supplementation, and hydrocortisone improved patient’s clinical condition and laboratory findings.