It had not been able to feed for up to three days after swimming in to the Thames.
Experts said it is likely to have swum into the North Sea by mistake and then headed up the Thames because it was trying to head west back to its feeding grounds to eat squid.
Scientists examined the whale's body for up to eight hours at the dockside in Denton, near Gravesend, Kent, and samples were then taken back to the Zoological Society of London (ZSL) for further analysis.
The ZSL veterinary scientists found the northern bottle-nose whale was a juvenile female which was probably less than 11 years old, sexually immature, measured 5.85 metres (19ft) in length and weighed several tons.
Dr Paul Jepson, ZSL veterinary pathologist, said: "Preliminary findings from the post mortem examination suggest that the ultimate cause of death is believed to be the result of a combination of factors including severe dehydration, some muscle damage and reduction of kidney function.
"This follows a period of up to three days in the Thames, an environment to which the animal is not suited.
"Whales and dolphins obtain water from their food and Northern bottle-nose 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."
Laboratory tests are continuing to see if the whale had any bacterial or viral infection.
Tests for pollutants such as heavy metals are also ongoing.
Dr Jepson added: "The last few days have been an unforgettable and ultimately sad experience for us all, and we are now determined that the whale did not die in vain.
"The incident has demonstrated a clear message of the nation's passion for these animals and their conservation.
"We are keen that all our learning from this experience is shared as widely as possible.
"Some tissues will go into the national marine mammal tissue bank, including that held at ZSL, providing a vital scientific resource for worldwide conservation biology."
Doctor Jepson said today's findings were only the initial result from the post mortem, and further tests would take several more weeks to complete.
He said numerous samples of tissue had been taken from the whale as it lay on Denton Wharf, near Gravesend in Kent, where it died.
The scientists spent between six and eight hours examining the animal before taking the samples to the ZSL's headquarters in Regents Park.
Speaking from the main offices today he dismissed speculation that the death of the whale could have been caused by anti-submarine sonar from Navy vessels.
"We think this is unlikely," he said.
Dr Jepson said studies suggested that sonars had affected pods of whales in the past, whereas this sea mammal was alone.
He also added that the number of whales stranded along the east coast of the UK was an "infrequent" but not unique experience.
Dr Jepson said the most likely reason for the whale ending up in the Thames was that it had somehow taken a wrong turn into the North Sea and was trying to head west to the north-east Atlantic by whatever means possible.
This would include travelling up river estuaries, through shallow waters and even passing human presence in order to travel west.
He described the whale's "innate sense of navigation" which would guide it through the water.
"While we do not know for sure that the whales are aware that they need to go west, it is one theory that has been proposed that makes sense."
Dr Jepson said the whale became dehydrated because it was not able to feed on its normal diet of squid, which live in the deep waters of the Atlantic.
Its main source of water comes through feeding on the squid.
But despite not eating for "several days", he said the whale was not starving and had not lost a significant amount of body mass.
The whale was also in good health as there was no significant evidence of parasites on it.
Becki Lawson, a veterinary scientist from ZSL, said the whale did not show any major signs of distress until about 6.30pm last Saturday, when its condition deteriorated badly.
She said the rescuers were monitoring the whale on a "minute-by-minute basis", checking its respiratory rate, its demeanour and general wellbeing.
She said: "Her condition deteriorated quickly and convulsions was part of that deterioration.
"Her main body systems were collapsing at that stage."
Dr Jepson said the decision had been taken to euthanase the whale for her own welfare, but she died before they could carry out the procedure.
"While I was drawing the lethal injection, she died," he said.
The veterinary scientist said they did not think there was anything more they could have done to try to rescue the whale.
Mark Stevens from the British Divers Marine Life Rescue group, said they had to get the whale on to the barge by 3pm last Saturday otherwise the water would have been too shallow to leave the Thames estuary.
Mr Stevens said if money had been no object he would have liked to have used bigger equipment to move the whale, but the BDMLR was a charity and funds were limited.
Over the next couple of weeks, further tests will be taken on the sea mammal as her bones will be given to the National History Museum in London.
Richard Sabin, from the museum, said the process of cleaning the bones had already started and the entire skeleton would be kept for scientific research.
He said scientists worldwide would be able to access the bones to help any further study.
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