Buenos Aires: President Cristina Fernandez has thyroid cancer, but test results show that it remains limited to a lobe in the right side of her neck, and has not metastasised or spread into her lymph nodes, her spokesman said.

Alfredo Scoccimarro said the cancer was discovered during a routine exam on December 22, and that Fernandez received the results on Tuesday from follow-up tests hours before the announcement.

Fernandez, 58, will undergo surgery on January 4 at the Hospital Austral in Buenos Aires and then take 20 days of medical leave, during which Vice President Amado Boudou will run the country.

Meanwhile, she will keep up her normal routine, Scoccimarro suggested, noting that she will appear at several events today as planned.

This kind of thyroid cancer is highly survivable, with more than 95 per cent of patients living at least 10 years after detection, according to the US National Institutes of Health.

The usual treatment is to surgically remove as much of the cancerous material as possible, and then follow up with radioactive iodide treatments, taken orally. This substance helps to destroy any remnants of the cancerous gland and provide for clearer images showing any additional cancer, the NIH said on its Web site.