As a result, Mrs. Clinton will not testify as scheduled on Thursday before Congressional committees investigating the September attack on the American diplomatic outpost in Benghazi, Libya.
The fainting episode occurred after Mrs. Clinton, who is being widely discussed as a possible presidential candidate in 2016, became dehydrated because of a stomach virus she contracted during a trip to Europe, according to statements released by Philippe Reines, a close adviser to Mrs. Clinton, and by her doctors.
“Secretary Clinton developed a stomach virus, leading to extreme dehydration, and subsequently fainted,” her doctors, Dr. Lisa Bardack of the Mount Kisco Medical Group and Dr. Gigi El-Bayoumi of George Washington University, said in their statement on Saturday. “Over the course of this week we evaluated her and ultimately determined she had also sustained a concussion.”
One State Department official said Mrs. Clinton fainted when she was alone at her home in Washington but added that the concussion was not diagnosed until Thursday. He said the concussion was not severe.
Acting on the advice of her doctors, Mrs. Clinton will not go to the State Department this week but will work from home, the State Department’s statement said.
William J. Burns and Thomas R. Nides, both deputy secretaries of state, will testify before the Senate Foreign Relations Committee in place of Mrs. Clinton, according to a spokesman for the panel. They are also expected to testify before a House committee about the attack, in which Ambassador J. Christopher Stevens and three other Americans were killed.
Besides the Congressional hearings, the State Department is preparing for an eventful week on the Benghazi attack, which had led to considerable partisan fighting about what precipitated the attack and what arrangements were made to defend the compound.
On Monday, an independent panel that was established to investigate the attack is expected to present its report to the State Department. The panel, called an accountability review board, is led by Thomas R. Pickering, a veteran diplomat. It includes four other members, including Mike Mullen, the retired admiral who formerly served as chairman of the Joint Chiefs of Staff. The board is authorized by a 1986 law intended to strengthen security at United States diplomatic missions.
The State Department plans to share the report with Congress and will also provide its own recommendations on how security for diplomats can be improved. Mr. Pickering and Admiral Mullen are expected to meet with lawmakers in closed sessions on Wednesday.
Then on Thursday, Mr. Burns and Mr. Nides will testify before the Senate committee, which is led by Senator John Kerry, the Massachusetts Democrat who is expected to succeed Mrs. Clinton as secretary of state.
Mrs. Clinton has said that she takes responsibility for the failure to successfully defend the Benghazi compound in the Sept. 11 attack. But she has never been questioned by lawmakers about how decisions were made by the Obama administration to establish the compound and protect it.
When a House oversight committee held a hearing on the Benghazi attack in October, the State Department was represented by a senior management official and a midlevel official from the department’s Bureau of Diplomatic Security. The former chief security officer for the embassy in Libya, Eric A. Nordstrom, told that panel that some of his requests for additional security were ignored. But the State Department’s under secretary for management, Patrick Kennedy, countered that none of the steps proposed by Mr. Nordstrom would have altered the outcome in Benghazi because the embassy was based in Tripoli.
The political debate over the Benghazi attack has already claimed one victim: the ambassador to the United Nations, Susan E. Rice. Ms. Rice had been the Obama administration’s top choice to succeed Mrs. Clinton as secretary of state, but last week she withdrew her name from consideration for the job because of the controversy over her initial description of the attack as a spontaneous demonstration that spun out of control.
On Tuesday, Mrs. Clinton canceled a planned trip to Morocco, where she was expected to formally recognize a new Syrian opposition coalition as the legitimate representative of the Syrian people. President Obama conveyed the recognition instead in an interview with ABC News.
Last week, State Department officials gave a mixed picture about the severity of Mrs. Clinton’s illness. On Wednesday, a State Department spokeswoman, Victoria Nuland, described Mrs. Clinton as having a “very uncomfortable stomach virus.”
The next day, Ms. Nuland said Mrs. Clinton was “under the weather.” Ms. Nuland did say that Mrs. Clinton’s illness had prevented her from making any calls to foreign leaders.