Washington (CNN) - Secretary of Labor Hilda L. Solis resigned her post Wednesday, offering no specific reason but that she "enjoyed my first opportunity in years to reflect on the past and my future" over the holidays.
"Leaving the department is one of the most difficult decisions I have ever made, because I have taken our mission to heart," she wrote in a letter to colleagues obtained by CNN. "As the daughter of parents who worked in factories, paid their union dues and achieved their goal of a middle class life, and as the first Latina to head a major federal agency, it has been an incredible honor to serve."
Solis provided no date when she would step down, but Labor Department officials told CNN her departure is expected around the inauguration, which is later this month.
"Over her long career in public service – as an advocate for environmental justice in California, state legislator, member of Congress and Secretary of Labor – Hilda Solis has been a tireless champion for working families," President Barack Obama wrote in a statement. "Over the last four years, Secretary Solis has been a critical member of my economic team as we have worked to recover from the worst economic downturn since the Great Depression and strengthen the economy for the middle class.
"Her efforts have helped train workers for the jobs of the future, protect workers' health and safety and put millions of Americans back to work. I am grateful to Secretary Solis for her steadfast commitment and service not only to the Administration, but on behalf of the American people. I wish her all the best in her future endeavors."
Her resignation comes amid other high-profile shifts at the top ranks of Obama's administration, including the upcoming resignations of Secretary of State Hillary Clinton, Treasury Secretary Timothy Geithner, Defense Secretary Leon Panetta.
She was one of four first term female cabinet secretaries. Solis' resignation just exasperates a diversity problem for the second term Obama cabinet. All four of the top cabinet jobs in the second term - State, Defense, Justice and Treasury - are expected to be occupied by males. White House Press Secretary Jay Carney said on Wednesday "diversity is important" and said the administration is working to make sure it is diverse.
The administration is looking for a woman to serve as the next Commerce Secretary, two sources familiar with the nomination process told CNN. John Bryson resigned from the post in June, and the acting commerce secretary is Rebecca Blank.
A White House official said Attorney General Eric Holder, Health and Human Services Secretary Kathleen Sebelius and Veterans Affairs Secretary Eric Shinseki are expected to stay in their current posts. The official did not give a timetable on how long the three might stay, but none of them is expected to depart in the near future.
Solis noted her accomplishments at the Labor Department in the letter, writing, "Growing up in a large Mexican-American family in La Puente, California, I never imagined that I would have the opportunity to serve in a president's cabinet, let alone in the service of such an incredible leader."
"Because of our work, more people are receiving the wages they are owed," she wrote.
Solis also touted her work in relation to the American Recovery and Reinvestment Act and Obama's health care law.
AFL-CIO President Richard Trumka said after Solis' announcement that she "brought urgently needed change to the Department of Labor, putting the U.S. government firmly on the side of working families.
"Under Secretary Solis, the Labor Department became a place of safety and support for workers," he wrote. "Secretary Solis's Department of Labor talks tough and acts tough on enforcement, workplace safety, wage and hour violations and so many other vital services. Secretary Solis never lost sight of her own working-class roots, and she always put the values of working families at the center of everything she did."
Trumka urged the president to pick a successor who "will continue to be a powerful voice both within the Obama administration and across the country for all of America's workers."
Before joining the Obama administration in 2009, Solis was a congresswoman from California with a particular interest in environmental issues. She was the first Latina in the California State Senate, according to her official biography. She had also worked in the administration of President Jimmy Carter and in the federal Office of Management and Budget's Civil Rights Division.