Shire , Britain's third-largest drugmaker, reported 24 percent third-quarter revenue growth, helped by robust demand for hyperactivity drugs in the back-to-school period and its medicines for rare diseases.

The company, which is growing faster than its Big Pharma rivals, posted revenue of $1.09 billion, ahead of analysts' forecasts, and earnings per American Depositary Share (ADS) of $1.28, which fell slightly short.

Shire said it was on track to meet the market's expectations for full-year forecasts, which have consistently increased in recent months.

Analysts have raised their 2011 forecasts for earnings per ADS to $5.22 from $5.05 three months ago, according to a company-supplied consensus.

Shire's rare diseases franchise has benefited from long-running manufacturing problems at U.S. biotech company Genzyme, bought earlier this year by Sanofi .

Genzyme is still struggling to get supplies of Gaucher disease drug Cerezyme back on track, and said last month that supplies would be limited until January. Full production of its Fabry medicine Fabrazyme has also been delayed.

Patients have switched to Shire's Vpriv and Replagal medicines, giving the British company a leading position in a small but highly lucrative market.

The group has also been boosted by strong demand for drugs to treat hyperactivity in the United States, where prescription sales rose about 9 percent in the third quarter, according to analysts.

Sales of Shire's amphetamine-based attention deficit hyperactivity disorder (ADHD) drug Vyvanse rose 32 percent to $200 million. The group wants to extend sales of the drug into European markets and new therapy areas such as depression.

It reported a successful Phase III ADHD trial of Vyvanse in Europe on October 21, paving the way for the drug to be filed for regulatory approval in the region.

Analysts at JPMorgan Cazenove have said that Vyvanse could secure half of the European ADHD market, with peak European sales by 2020 of $450 million.

Analysts, on average, had expected Shire, which has grown its drugs portfolio through acquisition, to report revenue of $1.07 billion and earnings per ADS of $1.30, according to Thomson Reuters and company-supplied data.

(Editing by Ben Hirschler)