A 74-year-old man, who is a lifelong smoker, is diagnosed with a small cell carcinoma of the lung. Following further investigation, it is discovered that he has developed the syndrome of inappropriate ADH secretion (SIADH) as a consequence of this.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by the excessive unsuppressible release of antidiuretic hormone (ADH), either from the posterior pituitary gland, or an abnormal non-pituitary source.
The typical biochemical picture seen in SIADH is:
- Low serum sodium (typically less than 135 mmol/l)
- Low serum osmolality
- High urine osmolality
2. What would you expect his serum osmolality to be?
Under normal circumstances, if the serum osmolality is low, you would also expect the urine osmolality to be low, as the kidneys will attempt to retain solute. In SIADH, however, the excess levels of ADH cause water retention without concomitant retention of solute. As a result, concentrated urine that is relatively high in sodium is generated despite low serum sodium levels.
There are a wide range of conditions that dysregulate ADH secretion in the central nervous system and can cause SIADH, including the following:
- CNS damage, e.g. meningitis, subarachnoid haemorrhage
- Paraneoplastic syndromes, e.g. small cell carcinoma of the lung
- Infection, e.g. atypical pneumonia, lung abscess, cerebral abscess
- Drugs, e.g. carbamazepine, TCAs, SSRIs
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