MadSci Network: Medicine

Re: What are the dangers re: use of THALLIUM in radiological testings?

Date: Fri Jun 23 16:28:55 2000
Posted By: Bernadette Baca, Health Physicist, Division of Reactor Safety
Area of science: Medicine
ID: 961360308.Me

First I'll present some "ELEMENTAL" information taken from "The Visual 
Elements Periodic Table" and "WebElements" 
( and 
( :   

Thallium is found in several ores, one of which is pyrites, used in the 
production of sulphuric acid. The commercial source of thallium is as a 
by-product of pyrites roasting in sulphuric acid production. It can also be 
obtained from the smelting of lead and zinc ores. Thallium is also present 
in manganese nodules found on the ocean floor.  Crude thallium is present 
as a component in flue dust along with arsenic, cadmium, indium, germanium, 
lead, nickel, selenium, tellurium, and zinc. 

The use of thallium is limited as it is a toxic element. Thallium sulphate 
is employed as a rodent killer - it is odorless and tasteless - but 
household use of this poison has been prohibited in the USA.  Thallium 
oxide is used to produce glasses with a high index of refraction, and also 
low melting glasses which become fluid at about 125K.

Elemental thallium has no known biological role. It is considered toxic and 
teratogenic. Contact of the metal with the skin is considered dangerous, 
and there is evidence that the vapor is both teratogenic and carcinogenic.

This is the "elemental" information readily available and present as "the" 
information to use in consideration of the harmful effects of thallium.

Now, here's the consideration of GREATEST importance.  The "compound" 
and/or molecular structure USED plays the biggest factor in many of the 
health and/or adverse reactions of any substance in question.  For example, 
there are many drugs on the market that produce medical wonders. However, 
if something in the manufacturing process should go wrong, and the 
"structure" of that compound changed, it could then very well become a 
fatal compound.  This is the nature of all man made substances.  Even 
though the "elemental" aspects of thallium say it's dangerous, it's the 
chemical structure that truly dictates whether or not, or more 
specifically, how dangerous a compound is.  One needs to be aware that the 
correct references and perspectives are looked at to ensure the safety of a 
particular substance.  And above all, make no assumptions on your own about 
unknown substances and compounds.  Always seek professional (medical) 
advice if you have ANY questions.  And ALL questions are valid questions if 
you should have any concerns; so ASK.  

In addition, the amount of the substance in question needs to be 
considered.  There is always the possibility for an unintended "overdose".  
Even with good things too much can be harmful; i.e. too many aspirin can 
create ulcers and too much water is called drowning.  The amount of 
thallium in the diagnostic drug is considered a "trace" quantity and not 
considered deleterious.

The main medical reasons thallium is used in many nuclear diagnostic tests 
is that the biological characteristics of thallium-201 is known to be 
similar to those of potassium as a transport substrate in the presence of 
glucose, insulin or phobolmyristate acetate (PMA).  This means it readily 
flows through the bloodstream and is utilized similarly to potassium.  The 
half life of thallium is relatively short (72 hours) and clears the body 
easily and quickly.  Thallium is superior to Tc-Sestamibi (once "the" 
diagnostic drug of choice to use) in the evaluation of myocardial viability 
because of several of it's unique properties.  One of which it is 
redistributed after some time (which also is used in a medical diagnosis) 
after administration.

Since you mentioned you were to receive a myocardial perfusion study, 
thallium stress test, I'll include some information about the important 
uses of thallium in these tests.  In myocardial perfusion imaging, 
thallium-201 is highly recognized for its ability to image "regional" 
myocardial blood flow distribution. Thallium-201 is very efficiently 
extracted by viable myocardial cells. After intravenous administration, 
thallium distributes in proportion to regional blood flow.  Images of the 
heart shortly after thallium administration show deficits in regions where 
blood flow is relatively reduced and in zones of nonviable myocardium(e.g., 
previous myocardial infarction).  These areas of "deficit" show up 
typically as dark areas in the image.  Over time, "redistribution" of 
isotope generally occurs in previously ischemic zones, that is, defects 
related to ischemic myocardium normalize or "fill in."   Defects related to 
infracted or scarred myocardium typically do not "redistribute" over time 
and remain fixed.  However, imaging at 24 hours or after reinjection of 
thallium-201 may show viable but hypoperfused segments not otherwise 
identified by a standard redistribution study performed at 3 to 4 hours 
after isotope injection.

Myocardial perfusion imaging is most commonly used in conjunction with 
exercise stress, with thallium-201 administered through an indwelling 
intravenous line at peak exercise.  The patient then exercises for an 
additional 30 to 60 seconds, and images are generally acquired immediately 
after and again 3 to 4 hours after thallium administration.  In patients 
with stable angina who are unable to exercise, pharmacologic "stress" 
(i.e., dobutamine, adenosine, or dipyridamole) has been used to induce 
myocardial hyperemia, with subsequent regional inhomogeneities in the 
perfusion pattern related to coronary stenoses.  Finally, in patients with 
unstable angina or acute myocardial infarction, a perfusion study can be 
performed at rest.  As with exercise, serial imaging can be performed after 
pharmacologic or rest thallium administration and demonstrate 
redistribution in regions of rest ischemia or underperfused but viable 

In the scheme of things, nuclear medicines strive for best utilization.  
Will the target compound or element behave in a specific/desired way?  Will 
it go to the areas of interest?  Are the radiations emitted sufficiently 
detectable?  How long will the "radioisotope" stay in the body?  Is the 
compound medically safe?  And as with all "drugs", there may be an allergic 
reaction to one or more components of the drug - that's just a fact of 
life.  Fortunately, there have been only rare documented cases of an 
allergic reaction to the thallium tagged drug.

Even though most information out on thallium may say it's a dangerous 
substance, when chemically combined and utilized correctly, it has become 
one of the many useful tools in diagnostic medicine.  It's all in the 
perspective and structure.

I hope this has helped allay some of your fears.  And thanks for asking 
your question.  I've included some websites that might also be helpful.  
Best wishes with your diagnosis.

Possible helpful websites:    (highly 
(medical article on heart studies - guidelines)

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