Chief Justice William Rehnquist, who has thyroid cancer, was reportedly hospitalized overnight on Tuesday for a fever.

News reports quote U.S. Supreme Court spokeswoman Kathy Arberg as saying that Rehnquist was taken by ambulance to a hospital in Arlington, Va. on Tuesday night.

Rehnquist was admitted for observation and tests, Arberg says in the reports.

It's unclear if the fever is related to Rehnquist's cancer.

Rehnquist is 80 years old. He has been a member of the U.S. Supreme Court since 1972, serving as chief justice since 1986.

What Is Thyroid Cancer?

There are about 18,000 cases of thyroid cancer annually in the U.S. (13,000 women and 4,600 men), accounting for about 1.1 percent of all cancer cases and about 1,200 deaths a year.

Most patients are cured of their disease or live many years with the disease.

The thyroid gland — located in the lower front of the neck at the base of the throat — has two lobes. The thyroid gland makes two important hormones that help the body function.

Thyroid cancers are usually found when a bump is seen in the neck. A biopsy will determine whether the nodule (bump) contains cancer. Only about 5 percent-10 percent of nodules are cancerous.

More Information on Thyroid Cancers

There are four main types of thyroid cancer:

Papillary Follicular Medullary Anaplastic

Details about Rehnquist's type of thyroid cancer have not been made available.

The majority of thyroid cancers are either papillary or follicular cancers. These are commonly called well-differentiated cancers. Both medullary and anaplastic are undifferentiated, a more aggressive type of cancer.

Aggressive Thyroid Cancers

Medullary thyroid cancer is uncommon — accounting for about 5% of all thyroid cancers. Unlike other types of thyroid cancer, it is difficult to treat and usually is not curable.

Treatment requires surgery to remove nearly all of the thyroid and removal of surrounding lymph nodes.

By the time patients are diagnosed as many as 50 percent have cancer that has already spread to other organs such as lymph nodes. About 10 percent of patients have cancer that has already spread to the lungs or liver.

Medullary thyroid cancer occurs in families in about 20 percent of cases. Hereditary medullary thyroid cancer often occurs with other types of hormone-producing tumors. This syndrome is called MEN - multiple endocrine neoplasia.

Anaplastic thyroid cancer is a highly aggressive cancer. It is very uncommon, representing less than 2 percent of all thyroid cancers. Fewer than 300 cases are reported annually in the U.S., according to the American Association of Clinical Endocrinologists.

This type of cancer can be seen primarily in patients older than 50. It starts as a rapidly expanding mass in the neck associated with signs of compression — hoarseness, shortness of breath, and difficulty swallowing. Duration of symptoms is generally short, lasting from weeks to a few months.

The average survival time for anaplastic cancer is six to nine months.

What Are the Symptoms of Thyroid Cancer?

Often, thyroid cancer has no symptoms and is found by chance at a doctor's visit. When symptoms are present, the cancer can appear as a gradually enlarging lump on the front part of the neck that moves when swallowing. Any lump in the neck should be brought to the attention of your health care provider.

What Are the Causes of Thyroid Cancer?

No one knows what causes thyroid cancer, but experts have identified many risk factors:

Exposure to large amounts of radiation (either from the environment or in those who have had radiation treatment for medical problems in the head and neck, such as acne or fungal infections of the face). The cancer may not occur until 20 years or longer after radiation treatment. Heredity (particularly for medullary thyroid cancer). Sex. Cancer of the thyroid is more common in women than in men.

By Brunilda Nazario, MD, reviewed by Michael W. Smith, MD

SOURCES: Associated Press. Supreme Court of the U.S.: "The Justices of the Supreme Court." National Institutes of Health. American Cancer Society. American Association of Clinical Endocrinologists.