CUPERTINO, Calif. — Apple Inc. co-founder and Chief Executive Steve Jobs said Wednesday he is taking a medical leave of absence until the end of June — just a week after the cancer survivor tried to assure investors and employees his recent weight loss was simply caused by a treatable hormone deficiency.

Jobs, 53, said in a letter last week that he would remain at Apple's helm despite the hormone deficiency, and said he had already begun the "relatively simple and straightforward" treatment for the problem. But in an e-mail to employees Wednesday, Jobs backtracked.

"During the past week I have learned that my health-related issues are more complex than I originally thought," he wrote.

Apple's chief operating officer, Tim Cook, will take over Jobs' responsibilities while he is on leave.

After-hours trading on Apple shares was halted. The Cupertino, Calif.-based company's stock had sank $2.38, or 2.7 percent, to close at $85.33.

Jobs announced in 2004 that he had undergone successful surgery to treat a very rare form of pancreatic cancer — an islet cell neuroendocrine tumor. The cancer is easily cured if diagnosed early. Jobs did not have a deadlier and more common form of pancreatic cancer called adenocarcinoma.