Which one of the following statements regarding lithium (Li) is FALSE?
a. It has a narrow therapeutic range
b. Lithium levels are increased by drugs which induce cytochrome p450
c. Serum lithium levels for monitoring should be measured 12 hours after the last dose
d. Lithium inhibits the action of anti-diuretic hormone (ADH)
e. Hypothyroidism is a frequent adverse effect of lithium therapy
The answer is b.
Lithium is a monovalent cation used in the treatment of bipolar disorder, depression and Schizoaffective disorders.
When administered orally it is fully absorbed from the gut with peak levels 4 hours after ingestion. It has a narrow therapeutic index and toxicity is common. Hence patients on lithium require monitoring with blood taken for serum lithium levels 12 hours after the last dose.
Lithium is not metabolised at all by the liver and so enzyme inducers and inhibitors have no effect on levels. It is, however, almost entirely eliminated via the kidneys so that pre-existing renal impairment as well as concomitant use of nephrotoxic drugs (diuretics, ACE inhibitors and NSAIDS) are causes of lithium accumulation and toxicity. Symptoms of lithium toxicity are mostly GI (nausea, vomiting and diarrhoea) and, with more serious poisoning, neurological (tremor, ataxia, confusion and coma).
Lithium inhibits the action of anti-diuretic hormone (ADH) in the collecting duct of the kidney and is a well-recognised cause of nephrogenic diabetes insipidus. It also causes hypothyroidism of varying degrees in around 1 in 5 patients.