Charlotte Cobbald, 17, had been spending time on her father's farm on 4 August last year.
The inquest in Bury St Edmunds heard she had been treated for anorexia and depression before her death.
Recording a narrative verdict, assistant coroner Yvonne Blake said she believed Miss Cobbald had acted on impulse.
There was not enough evidence to record a suicide verdict, Ms Blake said.
The inquest heard Miss Cobbald was being treated in a mental health unit in Colchester prior to her death.
On the day of her death, she was spending time away from the unit with her father Stephen.
Her mother Hilde and her father have campaigned to increase awareness of anorexia and gain more government funding for its treatment.
Speaking in September, Mr Cobbald said: "We had noticed a subtle change in her temperament and then by the autumn we noticed Charlotte was not eating as much as she used to.
"Slowly and surely it [anorexia] crept up on us and Charlotte."
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