In the study, which looked more than 4600 people, the researchers looked at the connection between thyroid levels (measured by thyroid stimulating hormone -- TSH -- levels) and Body Mass Index (BMI) -- a calculation that looks at height compared to weight, and categorizes people as underweight, normal weight, overweight, and obese.
What they found was a positive association between BMI and TSH -- meaning that BMI rose as TSH rose -- and a negative association between BMI and Free T4, meaning that as Free T4 (a measure of circulating thyroid hormone in the bloodstream) rises, BMI tends to drop.
Even among people who had a so-called normal TSH level, those with the high end of normal TSH levels -- TSH 4.5 -- weighed approximately 12 pounds more than those who had a TSH on the low end of normal, with a median TSH of 0.28.
Overall, researchers concluded that thyroid function -- even when within normal range is a factor that helps determine body weight, and even slightly elevated TSH levels are associated with an increase in the occurrence of obesity.
Interestingly, thyroid function has the same impact on BMI as physical activity!
The researchers also found that even small variations in thyroid function -- within the normal range for Free T4 -- may contribute to the regulation of body weight in a population. It's thought that the circulating thyroid hormone may affect the body's "sleeping energy expenditure." According to the researchers,
"In a population where physical activity has been gradually diminished, even a relatively small contribution to energy expenditure mediated through thyroid hormones may be enough to accomplish increases in BMI."The researchers concluded:
"...we suggest that differences in thyroid function within what is considered the normal range is associated with differences in BMI, caused by longstanding minor alterations in energy expenditure. This is more pronounced when mild hypo- or hyperthyroidism is present. The prevalence of such abnormalities in thyroid function are high and may be influenced by environmental factors. As small abnormalities in thyroid function are common, thyroid function may importantly influence the prevalence of obesity in a population."
Mary Shomon, About.com's Thyroid Guide since 1997, is a nationally-known patient advocate and best-selling author of 10 books on health, including the New York Times best-seller "The Thyroid Diet: Manage Your Metabolism for Lasting Weight Loss," "Living Well With Hypothyroidism: What Your Doctor Doesn't Tell You...That You Need to Know," "Living Well With Graves' Disease and Hyperthyroidism," "Living Well With Autoimmune Disease," "Living Well With Chronic Fatigue Syndrome and Fibromyalgia," and the "Thyroid Guide to Fertility, Pregnancy and Breastfeeding Success." Click here for more information on Mary Shomon.
Source: Knudsen N, et. al. "Small differences in thyroid function may be important for Body Mass Index and the occurrence of obesity in the population." J Clin Endocrinol Metab 2005 May 3