Does Underactive Thyroid Lead To Weight Gain—Or Vice-Versa?

with Arthur Schneider, MD, PhD

Low thyroid functioning is often blamed for weight gain. Your thyroid gland, a butterfly-shape organ in the front of your neck, makes hormones that control metabolism, weight, breathing, heart rate and many other functions.

But new research has raised questions about whether the link could go the other way, according to Cari Kitahara, PhD, MHS, an epidemiologist at the National Cancer Institute.

underactive thyroid and weight gain

Some study findings suggest that excess weight may be behind the slowing down of thyroid function instead of a sluggish thyroid leading to an increase in unanticipated weight gain. Dr. Kitahara presented findings of her investigation regarding the interrelationship between obesity, thyroid function and weight at the 86th annual meeting of the American Thyroid Association in Denver, Colorado.1

Understanding the link between weight and thyroid is crucial, she and others say, because obesity raises your risk of getting thyroid cancer. And if her research and that of others bears out, treating an overweight person with thyroid medication may be the wrong course of action, she says, if in fact the weight gain is what made the thyroid sluggish. The better approach, in some cases, may be weight loss.1

Unraveling the Thyroid Cancer-Obesity Links

Excess weight and thyroid cancer risk are linked, experts know. Dr. Kitahara and her team reconfirmed this recently, looking at 22 studies that had been published in medical journals and reanalyzing the results, publishing them in Thyroid. These studies included more than 830,000 men and more than 1.2 million women; all had submitted information on height, body mass index (BMI) and waist circumference. 2

The higher the BMI throughout adulthood, the higher the risk of getting thyroid cancer and dying from it, the new analysis found. 2

Experts have long thought that thyroid stimulating hormone (TSH), which tells your thyroid to make and release thyroid hormones, plays a role in thyroid cancer, since it influences the growth of thyroid cells. According to Dr. Kitahara, if someone has low thyroid function, their TSH is high, and the thyroid hormones known as T3 and T4 are low—and weight gain often occurs.

If someone has an overactive thyroid or hyperthyroidism, TSH is usually low, the T3 and T4 are high, and weight loss occurs.

For more clues about which way the link goes, Dr. Kitahara's team has looked at those with normal thyroid function (known medically as euthyroid patients). "Research on euthyroid individuals has produced conflicting results, which suggests a bi-directonality," she says.

Small variations in weight in persons with normal thyroid function can affect levels of thyroid hormones, Dr. Kitahara has found. "In euthyroid patients, weight loss lowers TSH and T3, while weight gain raises TSH and T3," she says.1

How small of a gain or loss might have an effect on thyroid hormones levels? Dr. Kitahara tells EndocrineWeb that an increase in body mass index (BMI) of about 5 units can be enough to affect thyroid hormones. For instance, a person 5'8" whose weight goes from 140 to 170 pounds would increase their BMI from about 21 to 26 (25 and above is termed overweight). 1,4 

Another finding of interest that lends support to the idea that the weight affects the thyroid hormones levels: There is limited evidence that thyroid hormone treatment leads to weight loss or other benefits in obese patients who have normal thyroid function, Dr. Kitahara says.1

Understanding Impact of Obesity on the Thyroid Gland

If an obese person with normal thyroid function has a higher TSH, Dr. Kitahara says, ''it may be a consequence [not a cause] of excess weight," she says. Research also suggests that weight loss interventions may be best to reduce the risk of developing a high TSH level.

Reducing TSH to normal levels may also lower thyroid cancer risk, she says, since TSH fuels the growth of thyroid cells and TSH levels are found to be high in thyroid cancer patients with aggressive tumors, she says. More research is needed in this area, however, she says.1,3

Perspective & Action Plan for Patients

The concept of excess weight leading to an underactive thyroid may sound backwards, says Arthur Schneider, MD, PhD, professor emeritus at the University of Illinois College of Medicine at Chicago, who is familiar with the new research. However, he says that physicians should consider the possibility that ''some subtle changes in thyroid [function] may be the result of obesity, not the cause of it."

In Dr. Kitahara's view: If you are obese with an elevated TSH, weight loss may be a better course of action if testing does not turn up any underlying thyroid disorders. She points to other research finding that when obese patients with normal thyroid function lose weight, their TSH and T3 levels decline—among other health benefits.1

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