Joanne's daughter was on the brink of womanhood. She had breasts, acne, teenage mood swings and was clearly just a few steps away from starting her period. Sounds normal - till you realise she was only two years old.

Yes, two years old.

The Wellington toddler has precocious puberty, a condition in which the hormones triggering puberty are activated earlier than usual.

Marie was 18 months old when her family began to suspect something was wrong. Her mother, Joanne, says Marie towered over other children the same age and she had small breasts.

"My sister, who was a nurse, said to me one day that Marie didn't smell like a baby, and I thought: no, that's right, she doesn't. She had a sort of body odour. So I went on the internet and typed in the words breasts, tall, body odour, and it came back with precocious puberty. I thought, oh my God."

A battery of medical tests quickly followed. An X-ray of Marie's bones showed they were older than her chronological age, a clear sign that she was growing too fast.

Another test revealed she had the hormones of a teenager racing through her body.

"She hadn't had a period or anything like it at that stage, but she was pretty close," says Joanne.

Doctors then told Joanne and her husband Andrew that they needed to give Marie an MRI scan to see if she had a brain tumour, which can cause precocious puberty in a child so young.

They found a benign tumour near the hypothalamus, and it was this that had triggered the onset of puberty. The tumour's location, close to the optic nerve, meant it would be difficult to remove surgically, and even if it could be taken out, there was no guarantee puberty would stop.

Joanne says her emotions were all over the place during this time.

"I went on a big knowledge trek and every time the doctors mentioned something, I'd look it up on the internet to find out everything I could. It was really scary. I sat in front of the computer and cried for a week when I found what she had.

"The biggest thing is the isolation. When you're taking your child for an MRI, thinking about the possibility of cancer, nobody else really understands unless they've been through it."

But there was good news: Marie's precocious puberty could be treated.

Monthly hormone injections, fully funded, and regular medical monitoring would stop Marie's premature puberty and slow down the rate at which her bones were growing.

Wellington Hospital paediatric endocrinologist Esko Wiltshire says puberty is generally defined as precocious if it begins before the age of eight in girls or nine in boys. At present, he's treating about eight patients for precocious puberty in the greater Wellington region, and though he doesn't know how many New Zealand children have the condition, he expects similar numbers in other parts of the country.

"My sister, who was a nurse, said to me one day that Marie didn't smell like a baby, and I thought: no, that's right, she doesn't. She had a sort of body odour. So I went on the internet and typed in the words breasts, tall, body odour, and it came back with precocious puberty. I thought, oh my God."

He says puberty starts when the hypothalamus in the brain signals the pituitary, known as the body's master control gland.

This then sends a message to the ovaries and testicles to begin making the hormones that will eventually lead to sexual maturity.

Curiously, Marie's treatment involves her being injected with large doses of the hormone responsible for puberty.

"It's the hormone that the hypothalamus normally makes to stimulate the pituitary to stimulate the ovaries," says Dr Wiltshire.

"In the body, that hormone is normally released in small pulses, and for the pituitary to respond to it, it has to be produced in those pulses. By giving a big dose of it that lasts a month, we're switching off those pulses. It's just a continuous signal to the pituitary, and that switches the pituitary off."

Joanne says Marie, now four, dreads the monthly injections.

"Because of her age, she doesn't understand what's happening to her. We've told her that her body has tried to grow too quickly and she has to have an injection to stop that because it wouldn't be good. "She spent the first year of treatment screaming every time the nurse came to the door. She's got a lot better about it, but as she's got older, she's said things like: 'Could you ring the nurse and tell her that it's not convenient today?' I always say: `No, sorry, we're booked in and we just have to do it'."

Marie now has her own set of syringes (minus needles) so she can pretend to give her toys injections.

Side effects such as mood changes, acne, or breast or testicle enlargement can accompany the medication, but Joanne says Marie has been largely unaffected. "After she started treatment, she did have a bleed like a little period for a few hours, and that was really shocking. She was still in nappies. Andrew took her nappy off and he said, oh God, look at this. We rang the doctor right away, and he said: `It's all right, you get that sometimes'."

The other main form of treatment comprises regular medical checks to make sure Marie's condition is well controlled and that she's not growing too tall too fast. Children with untreated precocious puberty grow rapidly as youngsters but stop growing after a few years, leaving a child who seemed set to be extraordinarily tall instead facing life as an adult who's extraordinarily short.

Dr Wiltshire says that Marie, without treatment, would probably have stopped growing at age five or six. Instead, the outlook is good. She will probably continue treatment till she's about 10 years old, at which point the injections may stop so that puberty can restart and proceed at a normal pace.

Joanne says she understands that Marie will be of normal height as an adult and her fertility will be unimpaired, as a result of her "miracle" treatment. She's thankful Marie's condition was detected while she was still very young, and she says that often the changes appear more obvious in girls.

"I know a little boy who's got it, and his grandmother says: 'Oh, he's just a big boy.' Well, he's a big boy now at four, but he won't be at eight unless he's treated.

"We're lucky we picked it up so early. There's always a little ongoing worry that something might crop up, but the prospects are good, and we don't want this to be the defining feature of (Marie's) life."

• For more information about precocious puberty: www.magicnz.org.nz
• Names have been changed in this article.