Polyuria and Thirst (OSCE focused)

Candidate Instructions:You are the medical student in GP clinic. A 45-year-old man presents with increased thirst and passing urine frequently. Take a focused history to explore possible causes. Do not examine at this stage.

Key Areas to Cover ✅

• Presenting complaint – onset, duration, fluid intake, nocturia.
• Associated symptoms – weight loss, blurred vision, fatigue, recurrent infections.
• Past medical history – hypertension, obesity, family history of diabetes.
• Medications – steroids, antipsychotics.
• Lifestyle – diet, alcohol, smoking, physical activity.
• Red flags – DKA symptoms (abdominal pain, vomiting, confusion, rapid breathing).

Examiner Prompts 💬

• “What investigations would you order for suspected diabetes?”
• “How would you distinguish type 1 from type 2 diabetes in this man?”

Differential Diagnoses 🔎

• Diabetes mellitus (type 1 or 2)
• Diabetes insipidus
• Hypercalcaemia
• Psychogenic polydipsia
• CKD with impaired concentrating ability

Mark Scheme (10 points) 📝

Teaching Commentary 📚

A diabetes OSCE is about recognising the classic osmotic symptoms (polyuria, polydipsia, weight loss). Always ask about complications (blurred vision, infections) and red flags (DKA). Examiners expect you to mention HbA1c and fasting glucose for diagnosis, and to consider type 1 vs type 2.

🧑‍⚕️ Case Examples - Polyuria & Polydipsia

• Case 1 (Type 1 Diabetes Mellitus): 🍬 A 17-year-old boy presents with 4 weeks of thirst, polyuria, and weight loss. He is fatigued and has new nocturia. Random glucose is 22 mmol/L and ketones are positive. Diagnosis: Newly diagnosed type 1 diabetes. Teaching point: Classic osmotic symptoms in a young patient → always check glucose and ketones; urgent insulin initiation is required.
• Case 2 (Type 2 Diabetes Mellitus): 📈 A 55-year-old overweight woman presents with several months of increasing thirst and urination, particularly at night. Fasting glucose is 12 mmol/L and HbA1c is 64 mmol/mol. Diagnosis: Type 2 diabetes. Teaching point: Gradual onset of osmotic symptoms in middle age, often with metabolic syndrome features; managed with lifestyle modification ± metformin.
• Case 3 (Diabetes Insipidus - Central): 💧 A 30-year-old man reports extreme thirst and passing >6 L of dilute urine daily. Serum sodium is high at 152 mmol/L, plasma osmolality raised, urine osmolality very low. Water deprivation test shows no concentration; desmopressin corrects it. Diagnosis: Central diabetes insipidus. Teaching point: Failure to secrete ADH → polyuria + polydipsia with hypernatraemia; responds to desmopressin.
• Case 4 (Diabetes Insipidus - Nephrogenic): 🧪 A 65-year-old man on long-term lithium presents with thirst and polyuria. Labs show high plasma osmolality, low urine osmolality, and no response to desmopressin during water deprivation test. Diagnosis: Nephrogenic diabetes insipidus (lithium-induced). Teaching point: Kidneys fail to respond to ADH; treatment includes stopping offending drug and considering thiazides or amiloride.
• Case 5 (Primary Polydipsia): 🚰 A 28-year-old woman with anxiety presents with passing >5 L urine/day. Labs show low-normal sodium and very dilute urine. Water deprivation test shows gradual rise in urine osmolality. Diagnosis: Primary psychogenic polydipsia. Teaching point: Distinguished from DI by normal ADH axis; behaviourally driven excessive fluid intake → hyponatraemia risk.