Lithium

This test measures the amount of lithium in the blood.

Lithium is used to treat manic-depressive disorders and the manic phase of affective disorders, including mania. The therapeutic window is relatively small. Therapeutic drug monitoring is useful to optimize dose and avoid toxicity.

Lithium is a naturally occurring element that is also one of the principal drugs used as a mood stabilizer and antipsychotic in people suffering from bipolar mood disorder. While this drug is very effective in treating this condition, it has a very narrow therapeutic range, which means that the level of lithium in the blood needs to be controlled in a range which is effective. Too much lithium can results in lithium toxicity, and too little lithium is be subtherapeutic.

This test is performed in following conditions such as:

- Ineffective treatment of bipolar mood disorder
- Drowsiness, lack of energy
- Dizziness
- Muscle weakness
- Lack of coordination
- Slurred speech
- Nausea, vomiting and/or diarrhea
- Confusion
- Irregular tremors or shaking
- Restlessness

Lithium is completely absorbed six to eight hours after oral administration. Since the onset of action is slow (5 to 10 days), parenteral administration is of no advantage. The plasma half-life is 17 to 36 hours, and this drug is eliminated almost entirely by the kidneys. Lithium clearance averages approximately 20% of creatinine clearance, but significant variability exists among patients.

Lithium ion is not protein bound, is distributed in total body water, and is concentrated in various tissues to different degrees. After a steady-state has been achieved, the lithium level in cerebrospinal fluid is about 40% of that in serum, and renal clearance for an individual remains relatively constant.

In general, a good correlation exists between the serum concentration of lithium ion and therapeutic efficacy and toxicity; however, some patients who show no therapeutic benefit have adequate serum concentrations of lithium but low erythrocyte concentrations. Since lithium works intracellularly, the erythrocyte concentration of the drug may be more relevant than levels in serum; therefore, in unresponsive patients, doses that produce higher than usual serum concentrations can be used if erythrocyte concentrations are lower.

Additional notes:

It is advisable to perform periodic plasma sodium determinations. Low plasma sodium levels are associated with lithium retention; high levels with lithium elimination. Varying degrees of nephrogenic diabetes insipidus have been reported to occur in 33% of lithium treated patients. Lithium significantly inhibits antidiuretic hormone induced water transport in kidney. Lithium interferes with solute and water absorption from the gastrointestinal system producing nausea, vomiting, diarrhea, and abdominal pain. These symptoms may occur at any time, at any serum level. They most commonly occur during early treatment stages and usually clear spontaneously or by adjustment of dosage. Chronic lithium administration has a goitrogenic effect on 4% of lithium treated patients, with or without hypothyroidism. In general, lithium administration results in slightly decreased serum T4 levels and transiently elevated levels of TSH in nearly 33% of these patients.

References:

Gelenberg AJ, Kane JM, Keller MB, et al. Comparison of standard and low serum levels of lithium for maintenance treatment of bipolar disorder. N Engl J Med. 1989 Nov 30; 321(22):1489-1493. PubMed 2811970

American Medical Association, Division of Drugs and Toxicology. Drug Evaluations Subscription. Chicago, Ill: AMA, Spring 1992.