Middle-aged adults whose thyroid gland is mildly underactive, but still functioning in the normal range, may be more prone to weight gain, a new study suggests.

The thyroid is a gland in the neck that produces hormones that regulate the body’s metabolism. In a disorder called hypothyroidism, the gland is underactive, causing symptoms such as fatigue, sensitivity to cold, dry skin and weight gain.

But it has been unclear whether thyroid function within the standard range has an effect on body weight.

In the current study, reported in the Archives of Internal Medicine, researchers looked at the relationship between body weight and levels of thyroid-stimulating hormone (TSH) in more than 2,400 middle-aged adults.

TSH is released by the brain to stimulate hormone production in the thyroid gland. Higher TSH levels in the blood indicate relatively lower activity in the thyroid.

In this study, men and women with relatively high, but still normal, TSH levels tended to weigh more at the outset than those with lower TSH concentrations.

Moreover, those whose TSH levels tipped upward over the next several years were more prone to weight gain.

“Our findings raise the possibility that modest increases in serum TSH concentrations within the reference range may be associated with weight gain,” write the researchers, led by Dr. Caroline S. Fox of the National Heart, Lung, and Blood Institute in Bethesda, Maryland.

It’s too soon, however, to start tinkering with thyroid hormones in order to treat obesity, editorialists comment.

Metabolism is governed by a complex interaction between the nervous system and hormone-producing glands. And while this system, including thyroid hormones, may influence weight and obesity risk, obesity also seems to affect the system, according to Drs. Roy E. Weiss and Rebecca L. Brown of the University of Chicago Medical Center.

Several studies, they note, have shown that excess fat tissue might directly affect TSH levels.

The study included 2,407 men and women who were an average of 48 years old when the study began. Among the women, the average weight for those with the lowest TSH levels was 142 pounds, versus 155 among those with the highest TSH levels; the corresponding figures for men were 182 pounds and 189 pounds.

Over the next 3.5 years, the group as a whole put on a few pounds. However, men and women whose TSH levels crept up tended to gain more.

Women with the highest TSH levels gained an average of 9.3 pounds more than women with the lowest TSH levels. The average weight gain in men with the highest TSH levels compared with those with the lowest levels was 4.2 pounds greater.

More research, according to Fox’s team, is needed to confirm the findings, and to understand why TSH levels are connected to weight.

SOURCE: Archives of Internal Medicine, March 24, 2008.