Nearly 1 in 8 women will develop a thyroid disorder at some point in their life.[1] And, for many women with thyroid dysfunction, managing weight is perhaps one of the greatest frustrations.

Many factors influence an individual’s weight – some are related to lifestyle choices, while others are related to the individual’s genetics and family history. Thyroid and metabolic hormones fall into the latter category and play an important role in regulating how much body fat a person stores.

Research has proved that thyroid hormones regulate the body’s metabolism, however the relationship between the two factors and how weight changes is complex and poorly understood.[2]

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Hormones of the Thyroid Gland

The thyroid is a small, butterfly-shaped gland located at the front base region of the neck (see Figure 1). It absorbs iodine and uses it to produce two specific hormones–thyroxine (T4) and triiodothyronine (T3). These hormones are then released into the bloodstream to ensure that all the cells in the body are working properly.

National Library of Medicine (NIH)

Figure 1. Front view of the thyroid gland at the base of the neck. [Image source: National Library of Medicine (NIH).]

T4 is the primary hormone produced in large quantities by the thyroid gland. However, it is considered a “pro-hormone” and is converted into its active form, T3, in organs like the kidney and liver. T3 drives processes like heat production, oxygen consumption and the metabolism of fat and carbohydrates.[3]

However, the thyroid gland doesn’t work alone to regulate metabolism. Instead, it operates together with the anterior pituitary gland and hypothalamus, both located in the brain, to form a self-regulated circuit called the hypothalamic-pituitary-thyroid axis (HPT axis). (If you want to learn more about how your hypothalamus interacts with your pituitary in this feedback loop, a medical explanation is reported in the article “Introduction to Thyroid Diseases”, which is founder here at the Knowledge Center.)[RC1] 

How Do Thyroid Hormones Affect Metabolism?

​Metabolism refers to a series of processes involved in breaking down food and converting it into energy for the body to function. Some people have a slow metabolism, while others have a high metabolism. How efficiently an individual’s metabolic processes run is partly determined by genetics and partly by hormones, including thyroid hormones that regulate the body’s basal metabolic rate (BMR).[1]

​BMR is the measure of the calories which the body burns or energy it uses at rest to perform basic life-sustaining functions, such as breathing, nutrient processing, blood circulation, and other functions.[5] Thyroid hormones bind to receptors inside the cell nucleus and activate specific genes to increase metabolism. This leads to increased oxygen consumption, respiratory rate, and body temperature.[6]

​In addition, depending on the body’s requirements, thyroid hormones either enhance the breakdown of stored nutrients (fats, carbohydrates, and proteins) to generate energy or promotes storage. [6]

Though thyroid hormones do not affect blood glucose levels directly, they do help the body to manage and consume glucose by increasing its absorption, production, storage, and breakdown for energy.[6]

What Happens With Thyroid Dysfunction?

Since the thyroid gland plays a central role in maintaining metabolism, energy balance, and hormonal signaling, its dysfunction creates a ripple effect throughout the body, often resulting in a cascade of health issues.

There are two main types of thyroid dysfunction–hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).

Hypothyroidism

An underactive thyroid gland doesn’t produce enough thyroid hormones, slowing metabolism. This reduces the number of calories the body burns, leading to weight gain even when there is no change in diet or physical activity. Other symptoms include cold intolerance, digestive issues, mood changes, dry skin and dry hair.

Hypothyroidism affects an estimated 4% to 10% of people worldwide, and women have about 4 times higher prevalence of this disorder than do men.[7] It is most commonly caused by autoimmune conditions like Hashimoto's thyroiditis, other medications (ex. lithium), iodine deficiency, surgical removal of the thyroid or exposure to radiation therapy.[8]

Hyperthyroidism

Hyperthyroidism affects approximately 1% of the population. It usually develops during the middle age and affects more women than men.[9]

In hyperthyroidism, the thyroid gland produces too much T3 and T4, causing metabolism to skyrocket. However, despite an increased appetite, the body burns more calories rapidly, leading to unexplained weight loss. In addition, people with hyperthyroidism experience heat intolerance, increased heart rate, high blood pressure, etc.

An overactive thyroid gland often stems from conditions like Graves’ disease (an autoimmune condition), thyroiditis, or non-cancerous growth on the thyroid gland. [10]

Why Is Losing Weight A Challenge?

Losing weight proves to be a challenge for most people, but it is especially hard for those diagnosed with hypothyroidism. Though there isn’t enough research on the exact mechanism behind this correlation, a low T3 level and hormone resistance may be two factors, in addition to salt and water retention. In general, once thyroid disorders are treated, the ability to gain or lose weight is similar to those without thyroid hormone disorders.

T3 delivers oxygen and energy to cells. Hypothyroid patients often have low T3 levels, which results in lower BMRs. When metabolism is low, people require lower calorie intake and more calorie burn to maintain or lower their body weight. [4] However, in general the data suggests that taking thyroid hormone replacement solely for weight loss is not recommended, in fact it can lead to serious harm due to increased heart rate with very minimal weight change.

Leptin and insulin are metabolic hormones released by fat cells and the pancreas, respectively. Known as the “satiety hormone,” leptin tells the hypothalamus when a person has eaten enough, signaling thyroid hormone production to burn fat.

When leptin levels are high (commonly seen in people with obesity), the body’s sensitivity to leptin signal drops significantly, resulting in a condition called leptin resistance.[11] This means the hypothalamus doesn’t receive proper signals about fullness and satiety. Hence, the brain senses a mode of “starvation” triggering it to conserve calories, which leads to an increased feeling of hunger and decreased calorie burn. Simultaneously, the thyroid gland slows down metabolism. Due to their increased appetite, people with leptin resistance may eat more but burn fewer calories, resulting in weight gain. When a person gains extra weight, their fat cells produce more leptin, continuing the cycle of leptin resistance.

The pancreas releases insulin after meals to stimulate glucose absorption into cells from the bloodstream. Just like with leptin resistance, when insulin levels continuously stay elevated (due to frequently consuming meals high in carbohydrates, fats, and sugar, and also caused by a lack of physical activity), the cells become less sensitive to insulin signals. This means the body requires more than normal insulin levels to keep blood sugar levels balanced.[12]

Higher insulin levels cause weight gain. Since it is a fat-building hormone, high insulin levels can also lead to fat accumulation around the abdominal organs located at the waist, which can give rise to highly inflammatory conditions like type 2 diabetes, and metabolic-associated steatotic liver disease (MASLD), previously known as nonalcoholic fatty liver disease (NAFLD), which affects about 1 in 4 people worldwide.[14]

In the US, over 38 million people (about 1 in 10 people) have diabetes, of which approximately 90% to 95% have type 2 diabetes. [13]

Conclusion

Thyroid dysfunction can feel like an uphill battle, especially when it disrupts metabolism and makes weight management challenging. However, with the proper treatment and lifestyle adjustments, it's possible to regain control. The key is to work with a healthcare provider to optimize thyroid hormone levels. Once the thyroid is well-managed, weight regulation may become more achievable and other treatments such as behavior change using strategies such as CBT (Cognitive Behavior Therapy) and/or medication can be considered to help control food intake, satiety, and level of physical fitness.