The Thyroid Influences Nearly Everything—But Is It the Culprit for Obesity?

Steps you can take to avoid age-related weight gain

By Michael Ormsbee, PhDFlorida State University
Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Many people have blamed their weight gain on a faulty thyroid hormone. Thankfully, the power to influence your body composition still rests in your hands. Professor Ormsbee explains.

Thyroid gland illustration
When thyroid hormones are within the normal range, your body is perfectly capable of maintaining a healthy body composition with regular exercise and good nutritional choices. Photo By Anatomy Image / Shutterstock

What Is the Thyroid?

The thyroid hormones are produced by the thyroid gland located in the front of your neck. The two hormones released from the thyroid gland are called thyroxine, or T4, and triiodothyronine, which is T3

T4 is produced in greater amounts, but T3 is the more biologically active thyroid hormone within the body. In other words, T3 actually gets the job done. 

“These unique messengers affect most bodily functions and influence nearly every tissue in your body throughout your entire life,” Professor Ormsbee said. “Frankly, it would be easier to discuss what they don’t do.” 

In short, the thyroid hormones regulate body temperature and are required for efficient metabolism, normal growth, and development, and for the actions of many other hormones. Without normal levels of thyroid hormones and growth hormone, for example, infants and children experience growth and developmental problems.

Because the thyroid hormones are involved in many metabolic processes, they also play a role in maintaining and increasing your resting metabolic rate and production of body heat in a process called thermogenesis. 

Heat production is another process that uses energy and can influence your body composition over time. Many of the bodily functions that thyroid hormones influence expend energy, and we need to replenish that energy by eating. 

Thyroid Concentrations for the Obese

Normally, T3 will increase energy expenditure after we eat or burn more calories, as our food is being broken down and transported to various cells within the body. This can help to regulate weight when the thyroid hormones are within a normal range. Thus, we need normal T3 levels to be healthy in a number of ways.

Take, for example, a study in which obese individuals reportedly had higher concentrations of T3, with increased energy expenditure and metabolic rate. This correlation may sound odd to you, since T3 increases calories burned. However, this increase in T3 in these obese subjects may be an adaptive response to prevent further weight gain, and T3 may be increased in an attempt to reduce the amount of energy being stored as fat.

On the other hand, T3 has been shown to be very low in underweight subjects and during starvation, and this is likely your body’s attempt to slow metabolism and conserve energy. These adaptations look like they are in place to reduce fat mass in the obese and preserve or increase fat in the underweight, helping people maintain healthy body weight. 

Unfortunately, some people do experience chronically low levels of T3 and T4, which is called hypothyroidism. In this circumstance, weight gain is likely. If the opposite occurs and T3 and T4 are chronically high, this is called hyperthyroidism and would likely result in weight loss.

Importance of Medication

Keep in mind that in otherwise healthy individuals who are taking synthetic thyroid hormones, your doctor has likely brought your blood levels of T3 and T4 into normal ranges. So if you are taking medication for diagnosed hypothyroidism—or low thyroid—then you don’t have your thyroid problem to blame for weight or fat gain anymore. 

Your levels are normal because of the thyroid medication that your physician has prescribed. 

“As one endocrinologist that I know stated, ‘The rate of obesity does not match the rate of thyroid disorder diagnoses,'” Professor Ormsbee said. “Your body is perfectly capable of maintaining a healthy body composition with smart exercise and quality nutrition so long as your thyroid levels are regulated.”

Body Composition Influences

Hormones, gender, and age all influence your body composition. We have known for some time that men naturally have more lean muscle compared to women, while women tend to have more total body fat. 

This is often explained by the location of body fat and the unique reproductive function needs of women. The higher level of fat and lower level of muscle in women typically means that women have metabolic rates that are about 5% to 10% lower than men of the same height and weight because muscle mass is more metabolically active. In addition, women are 5 to 8 times more likely to develop hypothyroidism as compared to men, which, if not treated, may lead to weight gain.

In a scientific paper on sex differences in body composition and insulin resistance, the authors of this review stated that men who tend to have more fat around their internal organs were more insulin resistant than women who tend to store their fat around their arms and legs. 

Thus, the health consequences of excess fat depend not only on the presence of body fat, but where that fat is located. However, as we age, our hormones also change, and this influences our muscle tissue and fat storage.

After menopause, for example, women gain more abdominal fat mass due to the decline in estrogen concentrations. It has also been shown that women burn fat less efficiently after menopause, both at rest and during exercise. 

The prevalence of hypothyroidism, or underactive thyroid, increases with age, too, affecting about 9% of men and women over 60 years old. This may have implications for weight gain in later years because, as you know, a decrease in thyroid hormones slows metabolic rate if it’s not treated with medication. 

Additionally, decreased muscle mass and strength occurs as we get older, resulting from a gradual loss—about 5% to 10% per decade—of the skeletal muscle after the age of 30.

However, it’s not all doom and gloom—these massive changes to body composition only occur if you choose not to exercise or eat properly. In fact, other evidence shows excellent muscle mass quality and function in lifelong exercisers and athletes.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.
Dr. Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.

About Kate Findley 437 Articles
Kate is a writer, novelist, and blogger living in Los Angeles. She has been writing for The Great Courses since 2017. It incorporates her two favorite things: writing and learning.