The Relationship Between Thyroid Hormones, Metabolism, and Weight

Though scientists do know that thyroid hormones regulate your body’s metabolism, the exact relationship between thyroid hormones, metabolism, and weight changes is a complex one that isn’t well understood.

When your thyroid isn’t producing enough hormones (hypothyroidism), your metabolism slows down. This reduces the number of calories your body burns and often leads to weight gain.

Conversely, when your thyroid is producing too many hormones (hyperthyroidism), your metabolism goes into overdrive, burning more calories than normal and likely resulting in weight loss.

However, this is a simplistic picture of the various factors that are involved in these processes.

The Role of Metabolism

Your metabolism involves a series of processes that break down food and convert it into the energy your body needs to run. It also determines how quickly (or slowly) your body uses calories. Some of how efficiently your metabolism runs is determined by genetics.

You burn calories through physical activity and by performing your daily activities. Your body also uses energy when you’re resting. The efficiency at which it does this is called your basal metabolic rate (BMR). Whenever you have excess energy—the result of eating more food than your body can use—this energy gets stored in fat cells and you gain weight.

The Role of Thyroid Hormones

Thyroid hormones heavily influence your metabolism, your body temperature, and your appetite regulation. Your thyroid works closely with your hypothalamus, a small area of your brain, to maintain both your weight and your energy balance—what’s left after you eat and your body uses the calories it needs.

How this happens isn’t exactly clear, but one thing is certain: It isn’t thyroid hormones alone that play a role in how much energy your body uses. Thyroid hormones work along with a number of other hormones, proteins, nuclear receptors, and chemicals, which is why it’s such a complicated process.

Research

Various studies on the relationship between thyroid hormones and weight gain have returned different conclusions. Most often, studies have shown that in the obese population, the following is true regarding thyroid hormones:

• Thyroid stimulating hormone (TSH) levels are either at the high end of the normal range or elevated.
• Free T3 (triiodothyronine) levels are high.
• Free T4 (thyroxine) levels are low.

However, some studies have reported these hormone levels to be normal or even decreased.

For instance, a 2014 study looked at a community-based sample of 1,944 adults with normal thyroid function who were followed for 11 years. The researchers found that TSH levels increased as weight increased over time in both males and females.

The study concluded that the association between TSH and weight gain doesn’t necessarily mean that one causes the other. Instead, the researchers proposed that there could be a third factor involved.

But a 2017 study examined 1,100 participants with normal thyroid function and found no link between changes in TSH and body mass index (BMI) after a follow-up period of 10 years. This study did find, however, that patients with lower levels of free T4 tended to have higher BMIs.

The variance in study results further illustrates how intricate the relationship between thyroid hormones and weight really is. The bottom line is that further research is needed.

Hypothyroidism and Weight

As discussed above, the simplistic explanation is that having an underactive thyroid often causes people to gain weight because their bodies stop using calories efficiently. A lot of this weight is excess water and salt. Typically, the more severe the hypothyroidism, the greater the weight gain, but usually it’s between 5 and 10 pounds.

Somewhere between 10% and 60% adults with obesity also have autoimmune hypothyroidism. Some studies have concluded that even mild hypothyroidism can lead to weight gain and changes in one's BMI.

Weight Loss With Treatment

There aren’t many scientific studies regarding how much weight is gained or lost once hypothyroid people are treated with the synthetic thyroid hormone levothyroxine, but the limited evidence that is available indicates that not everyone loses weight with treatment and in the ones who do, it’s limited.

The More Medication Myth

You may have heard that thyroid hormones can be used to lose weight, even in people without thyroid disease. While synthetic thyroid hormones have been used this way in the past, research has shown that most people gain all the weight back once they stop using them. Worse, being on thyroid hormones unnecessarily can result in serious side effects.

For people who are already using levothyroxine, increasing the dose above what’s needed can lead to more weight loss, but it’s usually not significant and those pounds are often gained back once the dose is decreased again.

This is also an unhealthy way to lose weight, as an increased dose is also associated with adverse reactions, including heart issues, insomnia, headaches, menstrual irregularities, and skin rash.

Hyperthyroidism and Weight

Many patients with an overactive thyroid end up losing weight. And in general, the more severe the hyperthyroidism, the more weight that is lost. But because metabolisms in these individuals are working harder and faster, they often feel hungrier and some may gain weight instead.

This usually reverses once your hyperthyroidism is treated. You may find that you gain back all the weight you lost, which is totally normal. If you eventually end up in a hypothyroid state, as many hyperthyroid patients do after thyroid surgery or radioactive iodine therapy, you may also run into the challenges of maintaining or losing weight.

The Weight Loss Challenge

Losing weight can be a challenge when you’re dealing with hypothyroidism. Researchers aren’t exactly sure why this is, but low T3 levels and hormone resistance issues may play a role.

In some cases, knowing how much to eat and discussing the best foods with a dietitian or nutrition professional can be a helpful first step.

Low T3 Levels

T3 is the active thyroid hormone at the cellular level, delivering oxygen and energy to cells, and it’s often low in hypothyroid patients. Lower T3 levels are associated with lower resting metabolic rates.

As mentioned previously, when your metabolism is lower, you need both lower calorie intake and more activity to burn calories in order to maintain your current body weight or lose weight. This can make losing weight extremely difficult.

Hormone Resistance

Another contributing factor to the weight loss challenge may be that hormone resistance problems often occur in people with thyroid disease, including leptin resistance and insulin resistance.

Leptin Resistance

Leptin is a hormone that’s released by your fat cells. In addition to maintaining energy balance and metabolism, leptin also tells your hypothalamus when you’ve had enough to eat, stimulating thyroid hormone production to burn fat.

When there’s too much leptin, which also happens when you’re obese, your body becomes less sensitive to leptin’s signals, resulting in leptin resistance. This means that your hypothalamus isn’t being properly told that you’re satisfied, so it goes into starvation mode, decreasing the number of calories you’re burning and telling you that you’re still hungry.

In the meantime, your thyroid slows down your metabolism as your appetite increases, you eat more, and you burn fewer and fewer calories, all resulting in weight gain. And the more pounds you put on, the more leptin your fat cells make, further enabling this cycle.

Insulin Resistance

Insulin is a hormone produced by your pancreas which keeps your blood sugar levels regulated. It works by telling your cells to absorb the excess sugar, or glucose, in your blood after you eat and to use it for energy.

As in leptin resistance, when your insulin levels are continually elevated, your cells become less sensitive to the signals insulin gives. This results in needing even more insulin to keep your blood sugar level steady. Having higher levels of insulin causes weight gain and raises your risk of developing type 2 diabetes.

Insulin is considered a fat-building hormone—it increases the amount of fat around the abdominal organs, which is highly inflammatory and can raise your risk of chronic diseases like type 2 diabetes. Studies show that both hyperthyroidism and hypothyroidism can create insulin resistance.

A Word From Verywell

Thanks to the complexities involved with hormones and metabolism, the reality is that if you have an underactive thyroid, you may have to work harder to lose weight than someone without a thyroid disorder.

If you’re continually hitting roadblocks, talk to your healthcare provider about optimal treatment. Once your treatment is optimized—which means that your thyroid levels are in the optimal range for you, not just in the normal reference range—you may find that it’s easier to lose weight.

According to the American Thyroid Association, when your thyroid hormone levels are normalized, you should be able to lose, maintain, and gain weight just like someone without thyroid disease. It may also help to increase your protein intake, get your glucose and leptin levels checked, adhere to a healthier diet, and strengthen your muscles to boost your ability to lose those extra pounds.

Above all, don’t give up! Hard work and determination can get you where you want to go.