Hyponatraemia
Hyponatraemia and hypoglycaemia are typical, and the potassium is usually high.
What our notes say
- Hyponatraemia and hypoglycaemia are typical, and the potassium is usually high. (Internal Medicine 4th Year Endocrinology CAT: 30 MCQs)
- What is the most likely cause of the hyponatraemia? (2 marks) (Chemical Pathology Semester 1 — Short Answer Questions)
- Explain why the urine osmolality is high despite the hyponatraemia. (2 marks) (Chemical Pathology Semester 1 — Short Answer Questions)
- Nausea, sexual dysfunction, insomnia or agitation at first, hyponatraemia (especially in older patients), increased bleeding risk. (Introduction to Psychopharmacology)
- Hyponatraemia, rash (HLA-B*1502 risk of Stevens-Johnson syndrome in some Asian populations), blood count changes. | (Introduction to Psychopharmacology)
- | Atypical | *Mycoplasma* (young, extrapulmonary features, cold agglutinins), *Legionella* (hyponatraemia, diarrhoea, confusion) | (Pneumonia)
Study material on Hyponatraemia
- Chemical Pathology Semester 1 — Short Answer Questions
- Endocrine & Renal Pathology
- Introduction to Psychopharmacology
- Internal Medicine 4th Year Endocrinology CAT: 30 MCQs
- Renal, Liver, Cardiac & Adrenal Function Tests for Year 3
- Chemical Pathology: Endocrine Cases, Pituitary & Thyroid Q&A
Practice questions on Hyponatraemia
A 65-year-old heavy smoker presents with hyponatraemia, low plasma osmolality and concentrated urine. The most likely cause is:
- Diabetes insipidus
- Ectopic ADH secretion
- Addison disease
- Hyperaldosteronism
- Primary hyperparathyroidism ✅ Answer: D
Answer: A. Hyponatraemia, low plasma osmolality, and concentrated urine are characteristic of SIADH (Syndrome of Inappropriate ADH secretion), which in a heavy smoker is often caused by ectopic ADH secretion from small cell lung carcinoma.
A 65-year-old man with small-cell lung cancer develops hyponatraemia (Na⁺ 118 mmol/L), urine osmolality 520 mOsm/kg and euvolaemia. Which ectopic hormone production BEST explains this?
- PTHrP
- ADH → SIADH
- Ectopic ACTH
- Erythropoietin
Answer: B. SIADH from ectopic ADH secretion is the classic paraneoplastic syndrome of SCLC, giving euvolaemic hyponatraemia with high urine osmolality and low serum osmolality. PTHrP causes hypercalcaemia; ectopic ACTH causes Cushing's features. ---