Shares of Amgen Inc. jumped nearly 5 percent Tuesday on heavy volume as billionaire hedge fund manager Dan Loeb disclosed he had upped his stake in the company and called for its breakup.

Loeb said his fund, Third Point LLC, is now one of the Thousand Oaks biotech’s “largest shareholders” in a letter sent to investors on Tuesday, though he was not specific. Third Point had 450,000 shares of Amgen stock, according to a June 30 regulatory filing.

Loeb said he expects Amgen shares to reach $249 in two years in the event of a breakup, which would represent about 80 percent growth in value. He noted that his company sees the most upside in Amgen splitting into two companies, which some analysts have recommended.

“We believe there are three ways to win in Amgen, depending on the path management takes from here,” Loeb’s letter stated. “We see the most upside, however, in the scenario where Amgen strategically separates into two standalone business, as we have encouraged management to consider.”

About 80 percent of Amgen’s $18.7 billion in revenue last year came from five legacy products. Analysts have suggested the company would be more successful separating its business into a low-spending and high-margin unit selling legacy drugs, and a high-spending, research and development firm that is focused on the future pipeline.

Loeb said another option for the company is to continue with its cost-cutting and strategic restructuring, which already appears to have been a hit with investors. Shares are up 25 percent this year counting Tuesday’s spike.

In July, the company announced it would lay off up to 2,900 workers, or nearly 15 percent of its global workforce, and consolidate on its sprawling 120-acre campus in Thousand Oaks.

In response to Loeb’s letter, Amgen issued a statement Tuesday afternoon.

“Amgen’s Board of Directors frequently receives input from shareholders, including ideas like those offered by Third Point. Amgen’s Board of Directors and management remain focused on addressing significant unmet medical needs for serious illnesses and driving value for all shareholders,” the statement said.

Amgen added that it will further update shareholders on the company’s strategic priorities and its restructuring plans at its business review on Oct. 28.

Loeb has been active in the L.A. market recently. He backed controversial L.A. supplement company Herbalife Ltd. late last year with a $350 million investment amid a campaign by rival hedge fund manager William Ackman to discredit the company as a pyramid scheme. Loeb cashed out for a profit in 16 days.

Amgen’s shares closed up $6.58, or 4.8 percent, to $144.09 on the Nasdaq. Some 8.6 billion shares traded hands, about two and a half times its average volume.