Lithium

Lithium is a light metal that is used specifically for the treatment and prophylaxis of  manic depressive disorders. The psycotrophic effect of lithium is not seen in healthy people. Lithium is usually taken orally, usually with meals or as a slow-release form. Absorption and distribution are slow, taking up to 10 to 12 hours for complete distribution of a dose. The plasma elimination half-life is 20 to 60 hours. Lithium is not metabolized and is excreted entirely by the kidneys.

The full therapeutic effect may not be seen for 7 to 10 days. After the initial steady state has been achieved, serum lithium levels should be measured daily to establish a mean concentration between 0.5 and 1.2 mmol/L. When the desired therapeutic level has been achieved, lithium should be measured once a week for 4 weeks and then every other month thereafter. Renal and thyroid function should be monitored periodically because lithium can cause hypothyroidism and renal dysfunction. Lithium therapy may increase total and ionized serum calcium and magnesium levels.

Bipolar and unipolar depressions are often successfully treated with lithium concentrations between 0.8 and 1.0 mmol/L, while manic patients often require higher concentrations of 0.9 to 1.4 mmmol/L. 

Excessive sweating, fever, weight reduction and low salt diets may increase lithium levels. Methyldopa, L-dopa, and diuretics may also increase lithium concentration.  

The lithium therapeutic range is narrow. Mild toxicity can be seen at concentrations above 1.5 mmol/L. Toxic symptoms include slurred speech, confusion, and ataxia. 

Trough levels should be drawn 8 to 12 hours after the last dose. Therapeutic (trough) range is 0.5 -1.2 mmol/L. Critical value for lithium is 1.7 mmoL. 

Serum lithium levels are measured with an ion-selective electrode. Specimen requirement is one red-top or gel-barrier tube of blood. Trough levels should be drawn before the next scheduled morning dose, 8 to 12 hours after the last dose.

References

Cousins DA et al. Lithium: past, present, and future, The Lancet Pscyhiatry,2020;7(3):222-224.

Bortolozzi A et al, New Advances in the Pharmacology and Toxicology of Lithium: A Neurobiologically Oriented Overview, 2024;76(3):323-357.