A blood test for thyroid cancer can detect persistent or recurrent disease even before doctors can find any trace of a tumor, according to a new study.
The findings suggest that people treated for the disease should be examined regularly for early signs of recurrence.
The study, by researchers at The Ohio State University ( OSU ) Comprehensive Cancer Center ( CCC ), examined how well a test for thyroid cancer can predict whether the disease will recur.
The findings were published in the Journal of Clinical Endocrinology & Metabolism.
The test measures a protein known as thyroglobulin ( Tg ), which is made by thyroid-cancer cells.
The measurement is taken after a person is injected with a relatively new drug known as thyrotropin alfa, or Thyrogen.
The drug allows Tg testing without the sometimes debilitating side effects of hypothyroidism that otherwise accompany the test when stimulation testing is done.
We were surprised to find that even with relatively low thyroglobulin levels, and even when there is no sign of a tumor, about 80 percent of patients had a recurrence of their cancer within three to five years, says first author Richard T. Kloos, associate professor of internal medicine and of radiology.
This indicates that we are detecting these tumors very early, and that time and diligence may be needed to find them.
The study also found that even when Tg levels are very low or undetectable, about 2 to 5 percent of patients still have a recurrence after three to five years.
Currently, some thyroid-cancer treatment guidelines say that these patients may never need further testing, but our data contradict that, says Kloos, co-director of the Thyroid Cancer Unit at the OSU CCC.
Thyroid cancer is usually treated surgically by removing the thyroid gland, followed by drinking radioactive iodine to kill any remaining cancer cells. Patients must then take synthetic thyroid hormone for life.
With all thyroid cells eliminated, the Tg level should be zero, and its presence later signals a possible return of the disease.
Sometimes, however, low levels of Tg can be present following treatment, and slowly decline over time.
In the past, Tg testing required that patients stop taking their synthetic thyroid hormone several weeks before the test.
That worked fairly well, except that some people became miserable after they stopped taking their synthetic thyroid hormone and became hypothyroid, Kloos says. Some patients claimed that they'd rather die of their disease than go through that regularly.
Withdrawing from thyroid hormone can cause fatigue, weight gain, constipation, mental dullness, lethargy, depression and other symptoms.
Thyrogen, approved for use in 1998, allowed people to have a stimulated Tg test and continue taking the synthetic hormone.
The study sought to help interpret the results of the Thyrogen-assisted Tg test. It involved 107 patients ( 88 women and 19 men; average age 36 years ) treated for papillary, follicular or Hurthle cell thyroid cancer. Following surgery and radioactive iodine treatment, the patients were injected with Thyrogen and tested for Tg levels between January 1999 and March 2001.
The patients were divided into three groups based on their Tg reading. Group 1 patients had Tg levels below 0.5 ng/dL, group 2 had Tg levels of 0.6 to 2.0, and Group 3 had Tg levels greater than 2.0.
After three to five years, the researchers found recurrent tumors in about 80 percent of the patients with Tg levels above 2.0, and in about 2 percent of those with Tg levels below 0.5.
An estimated 25,690 new cases of thyroid cancer are expected in 2005, with 19,190 of those expected to occur in women; 1,490 people are expected to die of the disease. In addition, about 330,000 living Americans have been treated for thyroid cancer, about 20 percent of whom are likely to have a recurrence.
Source: Ohio State University, 2005