When a 16-year-old girl’s hair had become so matted from not brushing, as a result of severe depression, she asked her hairdresser to cut it all off.

Instead, hairdresser Kayley Olsson spent 13 hours over two days restoring her hair – and the results are incredible.

Olsson said the Iowa teen told her she had been suffering severe depression for years and felt so worthless, she hadn’t wanted to brush her hair or get out of bed.  The result was a matted, knotted clump of brown hair that she could no longer get a comb through.

So as school was resuming after the summer break, photo day was coming up and she wanted to do something about it.

"When she walked in she told us just cut it all off. (She said) 'I can't deal with the pain of combing it out'. She called herself worthless for it," Olsson said in a Facebook post sharing the teen's story anonymously.

Not about to give up on the girl's hair as she had done, Olsson decided to start the painstakingly slow task of detangling her hair.

"It honestly broke my heart and we tried everything we could to keep this child's hair for her!"

After eight hours one day and five hours the next, the teen's hair was shiny, freshly-trimmed and knot-free.

The hairdresser said it was all worth it when the girl told her: "I will actually smile for my school pictures today. You made me feel like me again".

"We finally made this beautiful girl smile and feel like she IS worth something."

Olsson said she hoped the post and accompanying photos, which have been seen by more than 175,000 people since they were uploaded on Wednesday, acted as a lesson to anyone who didn't take mental health problems seriously.

"Parents, take it seriously. Don't just push your kids off and tell them to get over something they legitimately can't," she wrote.

"A child should never feel so worthless to not even want to brush their hair."

If you or anyone you care about is suffering from poor mental health, including depression, please contact Lifeline on 13 11 14.