The embattled Silicon Valley blood-testing company Theranos Inc. told its investors this week that it has secured a $100 million loan from Fortress Investment Group LLC, averting for now a possible bankruptcy filing as its remaining cash dwindled.

The loan is “subject to achieving certain product and operational milestones,” Theranos’s founder and chief executive officer, Elizabeth Holmes, told the company’s shareholders in an email Friday evening that was reviewed by The Wall Street Journal. A spokeswoman for the company...