A United States-based group WildAid says independent tests on shark fins bought in Bangkok showed that some had mercury levels up to 42 times higher than the safe limit for humans.
Wildlife groups say that up to one hundred million sharks are killed each year by fishermen who slice off the fin and dump the creatures back into the sea.
"We are alarmed by these results because this means sharks' fins being distributed in Thailand and other countries around the world are sometimes essentially poisoned without the consumer's knowledge," said Steven Gastler, co-director of WildAid, who released the report in Bangkok.
"Most of the fins we found with mercury are coming from Hong Kong, which also distributes to San Francisco, London...wherever there is a Chinatown," he added.
Global campaign
WildAid, which this month launched a global campaign to persuade governments to ban shark fishing in their territorial waters, said there were only 15 to 25 shark attacks on humans recorded worldwide each year.
That makes dead sharks more dangerous to human health than live ones, the group says.
WildAid tested 10 sharks' fin samples from three major dealers in Bangkok's Chinatown district in April and found high levels of mercury in all of them.
Many were also found to have been pumped full of as yet unidentified chemicals to increase their size, a sign the number of larger sharks in Asian waters is dwindling, the report said.
Pregnancy danger
State-run Thailand Institute of Scientific and Technological Research did the tests.
Tests in Hong Kong this year gave similar results, while the governments of Australia and New Zealand have recently issued public warnings to pregnant women about eating shark meat, Mr Gastler said.
Consumption of high levels of mercury is particularly dangerous for pregnant women and their babies as it blocks the natural process of nerve cell formation in the brain.