It is very unusual that you have NOT vaccinated your child. The possible connection between vaccines and autism is a very recent concern. What dangers were you concerned about 3 years ago?
It is also unusual to have very high levels of thallium ONLY. Do you have any idea where it came from?
This is what I pulled from the above article and a couple of other sources:
"Most cases of thallium toxicity occur after oral ingestion, but toxicity also may occur after inhalation of contaminated dust from pyrite burners,cadmium manufacturing, lead and zinc smelting, and even snorting what is believed to be cocaine. "
It seems it can also be from water systems associated with these industries.
Thallium has also been used for rat and ant poisons, photocells and infrared devices.
It is also uniquely and consistantly associated with hair loss.
The recommended treatment for thallium toxicity is chelation with Prussian blue and if the initial exposure to thallium is not identified and treated properly it can cause neurological damage - I haven't yet read anywhere that it can cause autistic behaviour, but maybe it can in the case of chronic low level exposure.
I think you should give some serious thought about where your child may have been exposed to thallium - especially to know if it was a single exposure that was not identified and treated or if it is an on-going chronic exposure.
It's hard to believe that you could have missed a single-time exposure, the reaction is severe and distinctive - vomitting, painful palms and soles of the feet and hair loss.
It's also hard to believe that it could be a chronic low level exposure unless you live near some metal manufacturing plant.