Thallium Toxicity Workup

Updated: Dec 12, 2021
  • Author: David Vearrier, MD, MPH; Chief Editor: Sage W Wiener, MD  more...
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Laboratory Studies

Because laboratory studies are generally nonspecific, any unexplained peripheral neuropathy, especially accompanied by alopecia, should raise clinical suspicion for thallium toxicity. In consultation with the medical toxicologist, initiate treatment of patients with high suspicion of thallium toxicity while awaiting laboratory confirmation.

The definitive clinical diagnosis of thallium poisoning can only be established by demonstrating elevated thallium levels. Thallium can be recovered in the hair, nails, feces, saliva, blood, and urine.

A 24-hour urine thallium concentration is the standard toxicologic method and is assayed by atomic absorption photospectrometry. The normal level is less than 5 mcg/L.

A urine spot test can deliver faster results. However, it often gives false-positive results, and it requires the use of 20% nitric acid, which can be dangerous and is usually not readily available.

Because thallium is rapidly distributed to the peripheral tissues, measurements of blood thallium reflect only recent exposures and may be falsely negative. Thus, they are not generally considered to be a reliable means of identifying or monitoring exposure to thallium.

A complete blood cell count (CBC) with differential can identify anemia, leukocytosis, eosinophilia, and thrombocytopenia, which have all been reported in cases of thallium exposure. However, none of these finding are sufficiently specific to make a diagnosis of thallium poisoning. 

Electrolytes, calcium, glucose, blood urea nitrogen (BUN), creatinine, and liver function tests (LFTs) should be obtained. Thallium exposure can lead to electrolyte and glucose abnormalities, hypocalcemia, and kidney and liver dysfunction.

A pregnancy test should be considered for all women of childbearing age. Thallium may cross the placental barrier and may be associated with fetal toxicity or mortality. [8]




Imaging Studies

Thallium is radiopaque; therefore, an abdominal radiograph should be obtained. This may reveal thallium metal after an acute ingestion. Radiographs of suspected exposure sources may be useful for confirming the presence of a heavy metal.

Liu et al investigated the correlation between functional imaging and long-term clinical imaging in cases of thallium toxicity. They concluded that fluorodeoxyglucose positron emission tomography (FDG/PET) imaging demonstrated the extent of brain involvement and correlated with cognitive impairment. [28]




Other Tests

Nerve conduction studies (NCS) may reveal findings consistent with an axonal sensorimotor peripheral neuropathy, with nerves innervating the feet most significantly involved. NCS may be useful in both diagnosing and monitoring patients with thallium exposure. The severity of abnormalities on NCS has been shown to correlate with the severity of other symptoms and findings.

An electroencephalogram (EEG) may show nonspecific slow-wave activity in severe cases.

Microscopic inspection of scalp hair reveals dark black and brown pigment in the hair roots in approximately 95% of poisoned patients. However, this may be difficult to visualize by the untrained observer. Darkening of the hair root can occur as early as 4 days postexposure. These dark regions are an optical phenomenon caused by the accumulation of gaseous inclusions that diffract the light, resulting in the appearance of a black band.

Electroretinographic (ERG) examination reveals a delayed visual evoked response. These ERG changes tend to occur before development of clinical symptoms in thallium intoxication. ERG may be useful when persons with known thallium exposure receive follow-up examinations. [29]

An electrocardiogram (ECG) should be obtained to identify tachycardia and cardiac arrhythmias.