Decreased thyroid function, or hypothyroidism, is commonly associated with weight gain, but a new study has found that effective treatment with levothyroxine (LT4) to restore normal thyroid hormone levels is not associated with clinically significant weight loss in most people.

Researchers SY Lee, LE Braverman, and EN Pearce describe the retrospective review of patients with newly diagnosed primary hypothyroidism over an 8-year period, not caused by thyroid cancer or other forms of disease or associated with pregnancy or use of prescription weight loss medication.

It was found that about 52 percent of the patients lost weight up to 24 months after initiation of treatment with LT4.

Ronald J. Koenig , M.D., Ph.D, Professor, Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor, said because obesity and hypothyroidism are very common, there are many patients who have both conditions.

Koenig said that these patients and sometimes their physicians often assume the hypothyroidism is causing the obesity even though this may not be the case.

This study is important because it shows, unfortunately, that only about half of hypothyroid patients lose weight after the successful treatment of their hypothyroidism.

The study will be presented at the 83rd Annual Meeting of the American Thyroid Association, in San Juan, Puerto Rico.