The whale was probably lost in the North Sea and trying to head back to the deep waters of the northeast Atlantic to find squid, experts said.
Jepson explained: "Whales and dolphins obtain water from their food and northern bottlenose whales normally feed on deep-water squid in the Atlantic Ocean.
"This animal would not have been able to feed while in the North Sea and so would have become dehydrated."
Jepson and his team of scientists examined the rare whale's body for up to eight hours at the dockside in Gravesend, downstream from London, and samples were sent back to the ZSL for further analysis.
The whale amazed onlookers by swimming past several London landmarks on Friday. Such a creature had not been seen in the Thames since records began in 1913.
As news of the bizarre sight spread, people flocked to the river to see the animal, which swam more than 40 miles (60 kilometers) from the open sea.
Jepson dismissed speculation that sonar from navy warships exercising in the North Sea could have disorientated the whale.
"Some scientists have speculated that northern bottlenose whales sometimes get into the North Sea by taking a wrong turn at Scotland," he said, outlining the most likely explanation for the whale's extraordinary journey.
"They use their innate sense of navigation to try and get back west to where they actually belong in the northeast Atlantic, often by perhaps trying to swim up rivers, despite shallow water and human presence."
Rescuers tried Saturday to return the whale to the North Sea, lifting it by crane onto a barge. However, it suffered convulsions and died halfway along the journey near Gravesend.
