Angela Mahoney told an employment tribunal that the strain of working as a regional official for the National Association of Schoolmasters Union of Women Teachers was so great that she feared it would worsen her epilepsy.
But the union denied that she was overworked, saying her 45- hour average week was slightly below the average worked by officials in other parts of the country.
At the time NASUWT was campaigning for a 35-hour week for teachers. It employed more regional office staff to relive the burden on its officials, but has yet to discover whether the changes reduced their hours.
Mrs Mahoney of Bury St Edmunds, Suffolk, alleged at a tribunal in the same town that the union had constructively unfairly dismissed her and subjected her to sexual discrimination.
While she was signed off sick, she claimed the union had discriminated against her by refusing to keep her on full pay. She would have been treated differently, she believed, if she had been a man.
But Jerry Bartlett, NASUWT assistant general secretary, said that only three members of staff had ever been given full long-term sick pay and that one of those had been a woman.
Mrs Mahoney was initially responsible for Essex, Suffolk, Norfolk and most of Cambridgeshire. A year later that was extended to include Hertfordshire, Bedfordshire, Luton and Peterborough, which she said represented a 58 per cent increase in members.
The situation had been made worse by the appointment of Tim Beech as regional organiser, who she alleged had bullied her and been constantly critical.
Mr Beech denied her claims. The amount of casework she was expected to handle for 14,500 members in the eastern region was less than that of other regional officials, he said.
Mrs Mahoney had tried to obstruct him, refused to accept he was in charge of the office and had acted beyond her authority, he told the tribunal.
Mr Bartlett claimed her behaviour in taking a job with Suffolk County Council in September 2002, while she was still considered to be on sick leave, amounted to serious misconduct. But Mrs Mahoney said she could not have returned to her post because of a breakdown in confidence in the union.
The tribunal is expected to deliver its judgment in June. The Association of Teachers and Lecturers said its officials were paid according to the level of their casework and worked an average of between six and 43 hours a week. The National Union of Teachers could not give an equivalent figure.