Thallium gastrointestinal toxicity

The relative toxicities of thallium compounds depend on their solubHities and valence states. Soluble univalent thallium compounds, eg, thaHous sulfate, acetate, and carbonate, are especiaHy toxic. They are rapidly and completely absorbed from the gastrointestinal tract, skin peritoneal cavity, and sites of subcutaneous and intramuscular injection. Tb allium is also rapidly absorbed from the mucous membranes of the respiratory tract, mouth, and lungs foHowing inhalation of soluble thallium salts. Insoluble compounds, eg, thaHous sulfide and iodide, are poorly absorbed by any route and are less toxic. 

Toxicology. Thallium is one of the most toxic of the heavy metals it primarily affects the nervous system and gastrointestinal tract and causes hair loss. 

Both T1(I) and T1(III) salts are readily absorbed by the gastrointestinal tract and the skin. Excretion is slow with a half-life of nearly one month. ThaUinm concentrates in the brain and testes. The lethal dose is less than 1 g of a thallium compound in a single ingestion. Thallotoxicosis involves the nervous system, skin, hair loss, and the cardiovascular system. T1+ can substitute for the similarly sized K+ (Table 2 and Figure 2) and interfere in K+-dependent processes (Figure 2). Recovery from thallotoxicosis takes months and may be incomplete as nervous system damage may be irreversible. Thallium may be the most toxic nonradioactive metal. 

Thallium and thallium salts are readily absorbed by virtually all routes, with gastrointestinal exposure being the most common route to produce toxicity. Thallium also crosses the placenta freely. Thallium enters cells by a unique process governed by its similarity in charge and ionic radius to potassium. Unlike potassium, however, once thallium enters the cells, it is released slowly. It can concentrate in the liver and kidneys. Since it is soluble at physiological pH, it does not form complexes with bone. Most thallium is excreted in the urine, but it is excreted slowly and can be detected months after exposure. 

Unlike exposure to most metal salts, gastrointestinal symptoms of thallium toxicity are relatively minor, and constipation is more characteristic than diarrhea. The major manifestations of toxicity consist of a rapidly progressive, ascending, extremely painful sensory neuropathy and alopecia. Other potential symptoms of overexposure are nausea, diarrhea, abdominal pain, and vomiting seizure, tremor and psychosis. Thallium is one of the most toxic of all metals. It is a cumulative poison with an estimated lethal dose of 8-20 mg kg in humans. It is difficult to predict the out come of thallium poisoning. With high exposure, death results very soon. 

The competitive replacement Tl /K creates an interesting toxicokinetic factor in terms of thallium elimination, namely direct active excretion into the intestinal lumen. In contrast to other toxic heavy metals, fecal elimination of thallium is the predominant route of excretion. In addition to thallium possibly binding in the gastrointestinal tract in cases of acute intoxication (cf. Section 22.6.2), reabsorption may occur by the enterohepatic and enterosystemic circulations, thereby prolonging the biological half-life. In fact, half-lives of between 3 and... 

---