Physiology
Endocrine
A patient presents with polyuria and polydipsia. Biochemistry results demonstrate a raised plasma osmolality and a low urine osmolality. Which of the following is the most likely diagnosis:
Answer:
Diabetes insipidus (DI) may result from a deficiency of ADH secretion (cranial DI) or from an inappropriate renal response to ADH (nephrogenic DI). As a result, fluid reabsorption at the kidneys is impaired, resulting in large amounts of hypotonic, dilute urine being passed with a profound unquenchable polydipsia. The biochemical hallmarks of DI are:- High plasma osmolality (> 295 mOsm/kg)
- Low urine osmolality (< 300 mOsm/kg)
- Hypernatraemia (> 145 mmol/L)
- High urine volume
Diabetes Insipidus
Physiology / Endocrine / Pituitary Function
Last Updated: 13th October 2023
Diabetes insipidus (DI) may result from a deficiency of ADH secretion (cranial DI) or from an inappropriate renal response to ADH (nephrogenic DI).
As a result, fluid reabsorption at the kidneys is impaired, resulting in large amounts of hypotonic, dilute urine being passed with a profound unquenchable polydipsia.
Biochemical Features
The biochemical hallmarks of DI are:
- High plasma osmolality (> 295 mOsm/kg)
- Low urine osmolality (< 300 mOsm/kg)
- Hypernatraemia (> 145 mmol/L)
- High urine volume (> 3L/24h)
Causes
- Cranial (ADH deficiency)
- Inflammatory hypophysitis
- Histiocytosis X
- Post-pituitary surgery
- Nephrogenic (ADH resistance)
- Metabolic or electrolyte disturbance
- Renal disease
- Drugs e.g. lithium
An ADH stimulation test can distinguish between cranial and nephrogenic DI, as nephrogenic DI shows an inability to concentrate urine even after administration of synthetic ADH.
Management
Patients with confirmed cranial DI should be investigated for pituitary disease, and managed as appropriate. Cranial DI typically responds well to synthetic ADH administration and results in good clinical improvement.
In nephrogenic DI, the underlying cause should be considered and reversed where possible. If symptoms persist, patients should drink according to thirst and keep up with water loss. Specific measures to treat nephrogenic DI include the use of low salt, low protein diet, diuretics and NSAIDs.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
