Alfredo Scoccimarro, the presidential spokesman, said that doctors detected the cancer during a medical examination on Thursday. He said there was "no existence of metastasis," suggesting that Mrs. Kirchner's chances could be strong for a recovery.

The announcement comes less than two months after Mrs. Kirchner, 58, glided to a re-election victory for a new four-year term. She has presided over an economy with remarkably robust growth rates, including a 9.3 percent expansion in the third quarter from the year earlier period, even as concerns persist over high inflation and capital flight.

Argentina was rocked last year by the death of Mrs. Kirchner's husband, Néstor, at age 60 from a heart attack. Mr. Kirchner had preceded her as president and led Argentina out of a severe economic crisis after being elected in 2003. He remained an unrivaled political operator during his wife's first term of office.

Mrs. Kirchner's illness is sure to reignite talk in the region about how various leaders have recently handled the disclosure of cancer diagnoses.

Others who have been stricken with cancer in Latin America include Brazil's former president, Luiz Inácio Lula da Silva, who is being treated for throat cancer, and his successor, President Dilma Rousseff, who was treated for lymphoma before being elected last year. Paraguay's president, Fernando Lugo, has also been treated in Brazil for non-Hodgkins lymphoma. President Hugo Chávez of Venezuela went into a long stretch of seclusion in Cuba this year before announcing in June that he had undergone surgery there for cancer.

Venezuela's government has never revealed which type of cancer afflicted Mr. Chávez, who said in October that he had recovered.

Mrs. Kirchner, like Mr. da Silva, was swift in announcing her diagnosis and provided details of her condition. Mr. da Silva was already out of office, but the news still came as a shock since he remains perhaps the most towering figure in Brazilian politics.

The papillary thyroid carcinoma found in Mrs. Kirchner is a common type of cancer, often beginning as a small bump in the thyroid gland in the neck, and the survival rate is generally high.

Mr. Scoccimarro, Argentina's presidential spokesman, said that Mrs. Kirchner would return to the presidency on Jan. 24. Vice President Amado Boudo will fill in during her absence, Mr. Scoccimarro said.