WASHINGTON — Sen. Robert C. Byrd, who holds the record as the longest-serving member of Congress, is seriously ill in a Washington-area hospital.

The West Virginia Democrat’s office says the 92-year-old lawmaker has been in the hospital since late last week.

At first he was believed to be suffering from heat exhaustion and severe dehydration. But his office says in a statement doctors have examined Byrd and other medical conditions have developed. His condition is described as serious.

The statement does not name the hospital.

In November, Byrd broke the record for congressional service that had been set by Democrat Carl Hayden of Arizona, who served in the House and Senate from 1912 to 1969.

Byrd began his career in Washington in 1952 with his election to the House. His elevation to the Senate came six years later.

Byrd has been in frail health in recent years and was hospitalized three times in 2009. He has been confined to a wheelchair, but was present and voted “yes” for final Senate passage of the health care reform bill in March.

Byrd has been the longest-serving senator since June 2006 and was elected to an unprecedented ninth term in November 2006. His colleagues have elected him to more leadership positions than any senator in history. He has cast more than 18,000 votes and has a nearly 98 percent attendance record over the course of his career.

His wife, Erma, died in 2006.