A constant battle with weight and illness has forced the premature retirement of apprentice Dylan Dunn.

Last season's champion Melbourne apprentice called a halt to his career on Tuesday after almost four years in the saddle.

The son of leading jockey Dwayne, Dunn wasn't born to be a jockey, standing 178cm and weighing 66kg before beginning his career.

After starting in Adelaide with Phillip Stokes, Dunn transferred to Lindsay Park in Melbourne where he was based last season.

Problems started to emerge with Dunn's health in late 2015 when he developed pneumonia in his left lung.

But it was being diagnosed with glandular fever in January this year that started Dunn thinking of a new career.

"I got really flat through the last part of last season which was basically from wasting so hard," Dunn said.

"I tried to push through it but when I was away in Queensland for the State Of Origin Series I knew had to do something about it.

"I was so lethargic. I couldn't get out of bed, didn't want to get out of bed so I went to the doctor, did some blood tests and it came back I had glandular fever."

Dunn said it went some way to explaining the fluctuations in his weight.

After riding at 55kg on a Saturday his weight would balloon to 60kg by Monday.

"It wasn't a fault of my own necessarily," Dunn said.

"I'd go out for a nice meal after the races and go home.

"I wasn't out partying.

"I'm only 22 and my body was starting to play up and I thought what's it going to be like in three, five, ten or 20 years time but I didn't expect to finish so soon."

To recuperate Dunn moved back to his parents' home on the Mornington Peninsula where he had the support of his girlfriend and school mates.

He didn't lie idle , completing a course in finance and setting up a mortgage brokering business.

In the future he hopes to take up an option to be a racing presenter while enjoying his new life.

"I was 66 kilograms before I was a jockey," Dunn said.

"I'm walking around at that weight now and I'm a lot healthier and a happier person.

"I wanted to walk out on a high and not be known as a battling jockey."

Dunn retires with 132 winners from 1170 rides and two Group Three winners to his name.

He counts riding Bold Sniper to victory at Sandown for The Queen as possibly his biggest highlight.