May 11 (Reuters) - Blood-testing firm Theranos Inc said it was adding three new members to its board of directors, and announced the retirement of its chief operating officer.

Theranos has been in the spotlight after reports last year suggested that the company was relying on traditional lab tools as it struggles with its own technology. It is currently being investigated by several U.S. regulators.

On Wednesday, Theranos said it would expand its board to include Dr. Fabrizio Bonanni, a former Amgen Inc and Baxter International Inc executive.

The other additions are Dr. William Foege, a former director of the U.S. Centers for Disease Control and Prevention, and Richard Kovacevich, former chief executive officer of Wells Fargo & Co.

Sunny Balwani, who played a key role in Theranos' product development, is also retiring as president and COO, the company said on Wednesday.

Palo Alto, California-based Theranos, which was founded in 2003 by Elizabeth Holmes, conducts a wide range of tests with one drop of blood from a finger-stick using its Nanotainers, rather than the large vial typically collected.

(Reporting by Natalie Grover in Bengaluru; Editing by Sandra Maler)