WASHINGTON—Facebook Inc.’s efforts to expand its reach among young people will attract scrutiny Thursday by members of the Senate Commerce Committee, who want to know more about the social-media giant’s research into how its products affect children.

The inquiry comes after Wall Street Journal reporting showing Facebook’s internal research found that its Instagram app was harmful for a sizable share of its millions of young users.

Lawmakers...

WASHINGTON— Facebook Inc.’s efforts to expand its reach among young people will attract scrutiny Thursday by members of the Senate Commerce Committee, who want to know more about the social-media giant’s research into how its products affect children.

The inquiry comes after Wall Street Journal reporting showing Facebook’s internal research found that its Instagram app was harmful for a sizable share of its millions of young users.

Lawmakers want to find out how Facebook executives responded when presented with the research, which concluded that photos and videos shared on Instagram made body image issues worse for a substantial minority of teen girls and that teens “blame Instagram for increases in the rate of anxiety and depression.”

“The more I know, the more repulsed I am by what I’ve seen,” said Sen. Richard Blumenthal (D., Conn.), who chairs the Commerce Committee’s consumer-protection panel that will conduct Thursday’s hearing. He said he and his staff have reviewed thousands of pages of Facebook documents and spoken with a person he identified as a whistleblower with knowledge of the issues.

“My focus is going to be, frankly, the revelations in The Wall Street Journal of the extraordinary danger and damage resulting from Facebook’s deep harm to young people, particularly girls,” he said in an interview. “I’m going to be asking Facebook why it knowingly concealed these facts and what it is going to do to correct these practices.”

Sen. Richard Blumenthal (D., Conn.) said he and his staff have reviewed thousands of pages of Facebook documents ahead of Thursday’s hearing.

Photo: Ken Cedeno - Pool via CNP/Zuma Press

Facebook has said the Journal’s reporting mischaracterized its internal research. “The research actually demonstrated that many teens we heard from feel that using Instagram helps them when they are struggling with the kinds of hard moments and issues teenagers have always faced,” the company said in a Sept. 26 blog post.

The Journal has defended its reporting, saying Facebook hasn’t cited a single factual error and was given ample opportunity to comment before publication.

Facebook will be represented at the hearing by Antigone Davis, the company’s global head of safety who is the sole witness scheduled for Thursday’s hearing.

Ms. Davis, a lawyer who joined Facebook in 2014 from the office of Maryland’s attorney general, has represented the company in discussions with external mental-health experts and other advocates who work to protect children online.

In prepared remarks for Thursday’s hearing, Ms. Davis said Facebook is committed to the “safety and well-being of the youngest people who use our services.”

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“This work includes keeping underage users off our platform…and partnering with product teams to address serious issues like child exploitation, suicide and self-harm, and bullying,” she said, according to a copy of the remarks viewed by the Journal.

Ms. Davis said the Facebook research cited by the Journal was conducted “to inform internal conversations about teens’ most negative perceptions of Instagram.”

“Our research showed that many teens who are struggling say that Instagram helps them deal with many of the hard issues that are so common to being a teen,” she said in the prepared remarks.

In a 2016 speech in New York, she cited Facebook’s tools designed to help prevent suicides and other harms, tearing up as she recalled a friend who had died by suicide.

Sen. Jerry Moran (R., Kan.) plans to ask Ms. Davis how the company responded “once its research found that a significant portion of teen girls using their platform experienced negative mental health effects,” according to a statement from his office.

Sen. Marsha Blackburn (R., Tenn.) has called for lawmakers to respond to ‘Big Tech’s pattern of choosing profit over the well-being of young users.’

Photo: Pool/Getty Images

Following the Journal’s Sept. 14 article, Facebook said this week that it would pause work on a version of Instagram it is designing for children under 13.

Sen. Marsha Blackburn (R., Tenn.), the top Republican on the Senate subcommittee, called the pause “a step in the right direction” but said Congress must respond to “Big Tech’s pattern of choosing profit over the well-being of young users.”

The Journal article was one in a series of reports based on internal Facebook documents. In another installment Wednesday, the Journal reported that Facebook’s efforts to build a youthful audience extended beyond Instagram.

Facebook and other big technology platforms have faced congressional scrutiny for more than a year over issues including perceived privacy abuses, content-moderation practices and competition concerns.

Facebook has emerged as a primary target. Many Democrats blame the company for allowing misleading content to proliferate. Many Republicans still regard Facebook as emblematic of the perceived risks of the internet, particularly for children. The social-media giant’s privacy practices also have drawn scrutiny from regulators, including a record $5 billion civil penalty from the Federal Trade Commission.

Some lawmakers also want to build a case for legislation expanding protections for children online. One pending bipartisan bill backed by some members of the Senate panel would expand existing restrictions on the online collection of personal data about children under 13, including by placing new limits on tracking teens aged 13 to 15 so that the protections cover teenagers.

Write to Ryan Tracy at ryan.tracy@wsj.com and John D. McKinnon at john.mckinnon@wsj.com