Thallium

Thallium (Tl) is a soft metal used industrially in the manufacture of optical glass, and as a catalyst in some organic chemistry laboratories. It is still used in some countries as a rodenticide. It readily oxidises and is highly soluble in water, yielding a highly toxic, tasteless solution which has been used historically as a poison.

In vivo, thallium is readily absorbed from the GI tract, and is thought to block the sodium/potassium pump within the nervous system, giving rise to characteristic neurological symptoms (peripheral neuropathy, developing into paraesthesia of the extremities, spreading to the trunk). Whole-body alopecia is a striking symptom of thallium toxicity. Thallium is excreted mainly through the biliary system, but undergoes extensive enterohepatic recirculation. Treatment of thallium poisoning with Prussian Blue [mixture of potassium hexacyanoferrate(II) and ferric hexacyanoferrate (II)], serves to interrupt the enterohepatic recirculation and aid excretion.

Thallium is usually measured in urine for an indication of exposure, though blood, serum or plasma may also be used in severe cases.