By Anthony J. Brown, MD

NEW YORK (Reuters Health) - Several factors that predict a favorable response to radioiodine treatment in patients with hyperactive thyroid, or "hyperthyroidism," have been identified by researchers in a study published in the current issue of the journal Clinical Endocrinology.

Radioiodine is a radioactive isotope of iodine often used to treat patients with hyperthyroidism. The isotope destroys thyroid tissue and stops the excess production of hormones that cause the symptoms such as, heart palpitations nervousness, excessive sweating, weight loss, and tremor.

Female gender, lower levels of free thyroid hormone T4, the absence of a goiter and treatment with a 600 megabecquerel (MBq) dose of 131-Iodine, are all independent predictors of cure with radioiodine for hyperthyroidism, new research shows.

Some of these factors, including a 600 MBq dose and female gender, also predict the development of underactive thyroid, or "hypothyroidism" 1 year later.

"This is the first study to identify a number of independent clinical and laboratory parameters to predict the probability of cure and the risk of development of hypothyroidism in patients receiving radioactive iodine to treat hyperthyroidism," lead author Dr. Kristien Boelaert, from the University of Birmingham, UK, told Reuters Health.

Boelaert and colleagues analyzed data for 1,278 patients who were diagnosed with hyperthyroidism at the Thyroid Clinic at the Queen Elizabeth Hospital, Birmingham, from 1984 to 2006. Most of the patients - 1,013 -- were women. Radioiodine therapy was given as a single dose of 185, 370 or 600 MBq.

The cure rate with the 600 MBq dose was 84.1 percent, significantly higher than the 74.9 percent and 63 percent rates with the 370 or 185 MBq doses, respectively. The hypothyroidism rate at 1 year was also higher with the 600 MBq dose: 60.4 percent versus 49.2 percent and 38.1 percent with the 370 and 185 MBq doses, respectively.

In addition to the 600 MBq dose and female gender, younger patients, the absence of a palpable goiter and eye involvement were independent predictors of hypothyroidism at 1 year.

Based on the findings, the authors were able to generate formulae that accurately predicted cure and risk of hypothyroidism after radioiodine treatment.

Boelaert said that clinical use of the predictors found in the study could help identify "patients who should receive higher (radioiodine) doses, thereby increasing the chance of cure in some and minimizing the risk of development of hypothyroidism in others."

She added that "further evaluation of these findings and the validity of these formulae are required in different patient populations around the world."

SOURCE: Clinical Endocrinology, January 2009.

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