Dannii Minogue reveals the secret pain behind her weight loss

But she has revealed her weight loss wasn't gym-related, and instead her tiny frame was down to a thyroid condition that made her lose so much weight she hardly had enough strength to pick up her 13-month-old son.

The 39-year-old told how she quit the British version of The X Factor to concentrate on motherhood after she was asked to go between Australia and the UK to film auditions.

The Australia's Got Talent judge said the stress just became too much - and even affected her relationship with partner Kris Smith.

Minogue said: 'My thyroid stopped working and I had appendicitis.

'Leading up to that I was getting thinner and thinner, and stressing because I didn't know what it was. Normally you put on weight if you have an under-active thyroid. But there is a percentage of people that have it reversed.

'It was very hard to concentrate, you can't sleep and I got very weak. They explain it to you as running a car without petrol.'

Minogue said she struggled so much that she 'was finding it hard to pick Ethan up and I was thinking: I'm a new mum, I know I'm tired, but surely it can't be this hard?'

She added to British Stella magazine: 'When you've just had a baby you think it must be normal tiredness. But it got to the point where I thought, 'This cannot be normal.' Poor Kris. He was thinking after getting through the pregnancy and the baby that everything would be just fine and he couldn't work out why I wasn't well. It was bloody difficult.'