By Mark Potter

LONDON (Reuters) - Shire (SHP.L: Quote, Profile, Research) said on Friday that 2008 sales of its biggest drug hope, Vyvanse for attention deficit hyperactivity disorder (ADHD), were likely to be at the lower end of its forecasts, hitting its shares.

Shares in the third-biggest drugmaker fell more than 5 percent, even as it met forecasts with a 5 percent rise in first-quarter underlying earnings and said its full-year revenue expectations for the whole company were unchanged.

Vyvanse is key to Shire's future as it is hoping to switch patients to it from its current ADHD best-seller, Adderall XR, which could face generic competition from April 2009.

Some analysts have been disappointed by Vyvanse's growth since its launch in July 2007, but Shire got a lift on Wednesday when U.S. regulators approved it as a treatment for adults.

"Based on existing prescription trends and the recently received approval for the adult indication, we expect Vyvanse sales to be at the lower end of the previously stated range of $350 (176 pounds) to $400 million," Shire said in a statement.

Analysts said the edging down of guidance was all the more surprising because of the earlier-than-expected approval to treat adults, and suggested Vyvanse was struggling to take patients from Adderall XR, which continues to grow strongly.

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"It's only a modest downgrade ... but they've got a history of guiding conservatively and then beating that guidance," said Panmure analyst Savvas Neophytou.