The executive director of the Pituitary Foundation, Kit Ashley, has resigned from the charity with immediate effect.
A statement from the foundation confirmed she had left after deciding she had "taken the Pituitary Foundation as far as she can and would be looking to further her career elsewhere".
A spokeswoman for the charity declined to comment further on her departure.
Ashley, who is also a novelist, joined the Pituitary Foundation in May 2005 after working for charities in the US.
The position of Terry Lloyd, the foundation’s chair, is also unclear. The foundation’s report and accounts, which were filed on 19 March, were signed off by acting chair Mike James.
According to the Companies House website, Lloyd was "terminated" then reappointed as a director of the foundation on four occasions between 4 February and 2 March.
The spokeswoman refused to comment on Lloyd’s position.
The charity has been involved in a long-running dispute with one of its former volunteers, Jon Danzig, who asked the Charity Commission to investigate the foundation after he used the Data Protection Act to obtain copies of derogatory emails sent by staff and trustees of the organisation to its members.
Last year, the commission decided not to open an investigation into the foundation, saying that the dispute about the emails was a personal matter.
But Danzig’s case is set to be reviewed by one of the commission’s independent complaints reviewers.